diff --git a/tests/test_api_health.py b/tests/test_api_health.py
new file mode 100644
index 0000000..c9a4a0b
--- /dev/null
+++ b/tests/test_api_health.py
@@ -0,0 +1,62 @@
+"""Health and version API tests."""
+
+from __future__ import annotations
+
+import pytest
+from fastapi.testclient import TestClient
+
+
+class TestHealthCheck:
+    """Test health check endpoints."""
+
+    def test_health_check_success(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify health endpoint returns 200."""
+        response = test_client.get("/health")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "status" in data
+        assert data["status"] == "ok"
+
+    def test_health_check_database_connection(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify DB connection check."""
+        # Basic health check doesn't necessarily check DB
+        # This is a placeholder for future DB health checks
+        response = test_client.get("/health")
+        
+        assert response.status_code == 200
+
+
+class TestVersionEndpoint:
+    """Test version endpoint."""
+
+    def test_version_endpoint_success(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify version information."""
+        response = test_client.get("/version")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "version" in data or "app_version" in data
+
+    def test_version_endpoint_includes_app_version(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify version format."""
+        response = test_client.get("/version")
+        
+        assert response.status_code == 200
+        data = response.json()
+        # Version should be a string
+        version_key = "version" if "version" in data else "app_version"
+        assert isinstance(data[version_key], str)
+
diff --git a/tests/test_api_jobs.py b/tests/test_api_jobs.py
new file mode 100644
index 0000000..9a22d1f
--- /dev/null
+++ b/tests/test_api_jobs.py
@@ -0,0 +1,72 @@
+"""Medium priority job API tests."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+from fastapi.testclient import TestClient
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+
+class TestJobStatus:
+    """Test job status endpoints."""
+
+    def test_get_job_status_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 for non-existent job."""
+        response = test_client.get("/api/v1/jobs/nonexistent-job-id")
+        
+        assert response.status_code == 404
+        data = response.json()
+        assert "not found" in data["detail"].lower()
+
+    def test_get_job_status_includes_timestamps(
+        self,
+        test_client: TestClient,
+        test_db_session: "Session",
+    ):
+        """Verify timestamp fields."""
+        # This test requires a real job to be created
+        # For now, we'll test the error case
+        response = test_client.get("/api/v1/jobs/test-job-id")
+        
+        # If job doesn't exist, we get 404
+        # If job exists, we should check for timestamps
+        if response.status_code == 200:
+            data = response.json()
+            assert "created_at" in data
+            assert "updated_at" in data
+
+
+class TestJobStreaming:
+    """Test job streaming endpoints."""
+
+    def test_stream_job_progress_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 handling."""
+        response = test_client.get("/api/v1/jobs/stream/nonexistent-job-id")
+        
+        # Streaming endpoint may return 404 or start streaming
+        # Implementation dependent
+        assert response.status_code in [200, 404]
+
+    def test_stream_job_progress_sse_format(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify SSE format compliance."""
+        # This test requires a real job
+        # For now, we'll test the not found case
+        response = test_client.get("/api/v1/jobs/stream/test-job-id")
+        
+        if response.status_code == 200:
+            # Check Content-Type for SSE
+            assert response.headers.get("content-type") == "text/event-stream"
+
diff --git a/tests/test_api_people.py b/tests/test_api_people.py
new file mode 100644
index 0000000..fdb373e
--- /dev/null
+++ b/tests/test_api_people.py
@@ -0,0 +1,264 @@
+"""High priority people API tests."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+from fastapi.testclient import TestClient
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+    from backend.db.models import Person, Face, User
+
+
+class TestPeopleListing:
+    """Test people listing endpoints."""
+
+    def test_list_people_success(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+    ):
+        """Verify people list retrieval."""
+        response = test_client.get("/api/v1/people")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        assert "total" in data
+        assert len(data["items"]) > 0
+
+    def test_list_people_with_last_name_filter(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+    ):
+        """Verify last name filtering."""
+        response = test_client.get(
+            "/api/v1/people",
+            params={"last_name": "Doe"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        # All items should have last_name containing "Doe"
+        for item in data["items"]:
+            assert "Doe" in item["last_name"]
+
+    def test_list_people_with_faces_success(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+        test_face: "Face",
+        test_db_session: "Session",
+    ):
+        """Verify people with face counts."""
+        test_face.person_id = test_person.id
+        test_db_session.commit()
+        
+        response = test_client.get("/api/v1/people/with-faces")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        # Find our person
+        person_item = next(
+            (item for item in data["items"] if item["id"] == test_person.id),
+            None
+        )
+        if person_item:
+            assert person_item["face_count"] >= 1
+
+
+class TestPeopleCRUD:
+    """Test people CRUD endpoints."""
+
+    def test_create_person_success(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify person creation."""
+        response = test_client.post(
+            "/api/v1/people",
+            json={
+                "first_name": "Jane",
+                "last_name": "Smith",
+            },
+        )
+        
+        assert response.status_code == 201
+        data = response.json()
+        assert data["first_name"] == "Jane"
+        assert data["last_name"] == "Smith"
+        assert "id" in data
+
+    def test_create_person_with_middle_name(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify optional middle_name."""
+        response = test_client.post(
+            "/api/v1/people",
+            json={
+                "first_name": "Jane",
+                "last_name": "Smith",
+                "middle_name": "Middle",
+            },
+        )
+        
+        assert response.status_code == 201
+        data = response.json()
+        assert data["middle_name"] == "Middle"
+
+    def test_create_person_strips_whitespace(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify name trimming."""
+        response = test_client.post(
+            "/api/v1/people",
+            json={
+                "first_name": "  Jane  ",
+                "last_name": "  Smith  ",
+            },
+        )
+        
+        assert response.status_code == 201
+        data = response.json()
+        assert data["first_name"] == "Jane"
+        assert data["last_name"] == "Smith"
+
+    def test_get_person_by_id_success(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+    ):
+        """Verify person retrieval."""
+        response = test_client.get(f"/api/v1/people/{test_person.id}")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == test_person.id
+        assert data["first_name"] == test_person.first_name
+
+    def test_get_person_by_id_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 for non-existent person."""
+        response = test_client.get("/api/v1/people/99999")
+        
+        assert response.status_code == 404
+
+    def test_update_person_success(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+    ):
+        """Verify person update."""
+        response = test_client.put(
+            f"/api/v1/people/{test_person.id}",
+            json={
+                "first_name": "Updated",
+                "last_name": "Name",
+            },
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["first_name"] == "Updated"
+        assert data["last_name"] == "Name"
+
+    def test_update_person_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 when updating non-existent person."""
+        response = test_client.put(
+            "/api/v1/people/99999",
+            json={
+                "first_name": "Updated",
+                "last_name": "Name",
+            },
+        )
+        
+        assert response.status_code == 404
+
+    def test_delete_person_success(
+        self,
+        test_client: TestClient,
+        test_db_session: "Session",
+    ):
+        """Verify person deletion."""
+        from backend.db.models import Person
+        from datetime import datetime
+        
+        # Create a person to delete
+        person = Person(
+            first_name="Delete",
+            last_name="Me",
+            created_date=datetime.utcnow(),
+        )
+        test_db_session.add(person)
+        test_db_session.commit()
+        test_db_session.refresh(person)
+        
+        response = test_client.delete(f"/api/v1/people/{person.id}")
+        
+        assert response.status_code == 200
+
+    def test_delete_person_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 for non-existent person."""
+        response = test_client.delete("/api/v1/people/99999")
+        
+        assert response.status_code == 404
+
+
+class TestPeopleFaces:
+    """Test people faces endpoints."""
+
+    def test_get_person_faces_success(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+        test_face: "Face",
+        test_db_session: "Session",
+    ):
+        """Verify faces retrieval for person."""
+        test_face.person_id = test_person.id
+        test_db_session.commit()
+        
+        response = test_client.get(f"/api/v1/people/{test_person.id}/faces")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        assert len(data["items"]) > 0
+
+    def test_get_person_faces_no_faces(
+        self,
+        test_client: TestClient,
+        test_person: "Person",
+    ):
+        """Verify empty list when no faces."""
+        response = test_client.get(f"/api/v1/people/{test_person.id}/faces")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        # May be empty or have faces depending on test setup
+
+    def test_get_person_faces_person_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 for non-existent person."""
+        response = test_client.get("/api/v1/people/99999/faces")
+        
+        assert response.status_code == 404
+
diff --git a/tests/test_api_photos.py b/tests/test_api_photos.py
new file mode 100644
index 0000000..b57d72a
--- /dev/null
+++ b/tests/test_api_photos.py
@@ -0,0 +1,429 @@
+"""High priority photo API tests."""
+
+from __future__ import annotations
+
+from datetime import date
+from typing import TYPE_CHECKING
+
+import pytest
+from fastapi.testclient import TestClient
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+    from backend.db.models import Photo, Person, Face, User
+
+
+class TestPhotoSearch:
+    """Test photo search endpoints."""
+
+    def test_search_photos_by_name_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        test_person: "Person",
+        test_face: "Face",
+        test_db_session: "Session",
+    ):
+        """Verify search by person name works."""
+        # Link face to person
+        test_face.person_id = test_person.id
+        test_db_session.commit()
+        
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "name", "person_name": "John Doe"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        assert "total" in data
+        assert len(data["items"]) > 0
+
+    def test_search_photos_by_name_without_person_name(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 400 when person_name missing."""
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "name"},
+        )
+        
+        assert response.status_code == 400
+        assert "person_name is required" in response.json()["detail"]
+
+    def test_search_photos_by_name_with_pagination(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        test_person: "Person",
+        test_face: "Face",
+        test_db_session: "Session",
+    ):
+        """Verify pagination works correctly."""
+        test_face.person_id = test_person.id
+        test_db_session.commit()
+        
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={
+                "search_type": "name",
+                "person_name": "John Doe",
+                "page": 1,
+                "page_size": 10,
+            },
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["page"] == 1
+        assert data["page_size"] == 10
+        assert len(data["items"]) <= 10
+
+    def test_search_photos_by_date_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+    ):
+        """Verify date range search."""
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={
+                "search_type": "date",
+                "date_from": "2024-01-01",
+                "date_to": "2024-12-31",
+            },
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+
+    def test_search_photos_by_date_without_dates(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 400 when both dates missing."""
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "date"},
+        )
+        
+        assert response.status_code == 400
+        assert "date_from or date_to is required" in response.json()["detail"]
+
+    def test_search_photos_by_tags_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        test_db_session: "Session",
+    ):
+        """Verify tag search works."""
+        from backend.db.models import Tag, PhotoTag
+        
+        # Create tag and link to photo
+        tag = Tag(tag="test-tag")
+        test_db_session.add(tag)
+        test_db_session.flush()
+        
+        photo_tag = PhotoTag(photo_id=test_photo.id, tag_id=tag.id)
+        test_db_session.add(photo_tag)
+        test_db_session.commit()
+        
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "tags", "tag_names": "test-tag"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+
+    def test_search_photos_by_tags_without_tags(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 400 when tag_names missing."""
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "tags"},
+        )
+        
+        assert response.status_code == 400
+        assert "tag_names is required" in response.json()["detail"]
+
+    def test_search_photos_no_faces(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+    ):
+        """Verify photos without faces search."""
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "no_faces"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+
+    def test_search_photos_returns_favorite_status(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        admin_user: "User",
+        test_db_session: "Session",
+    ):
+        """Verify is_favorite field in results."""
+        from backend.db.models import PhotoFavorite
+        
+        # Add favorite
+        favorite = PhotoFavorite(photo_id=test_photo.id, username=admin_user.username)
+        test_db_session.add(favorite)
+        test_db_session.commit()
+        
+        response = test_client.get(
+            "/api/v1/photos",
+            headers=auth_headers,
+            params={"search_type": "date", "date_from": "2024-01-01"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        if len(data["items"]) > 0:
+            # Check if our photo is in results and has is_favorite
+            photo_ids = [item["id"] for item in data["items"]]
+            if test_photo.id in photo_ids:
+                photo_item = next(item for item in data["items"] if item["id"] == test_photo.id)
+                assert "is_favorite" in photo_item
+
+
+class TestPhotoFavorites:
+    """Test photo favorites endpoints."""
+
+    def test_toggle_favorite_add(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        admin_user: "User",
+        test_db_session: "Session",
+    ):
+        """Verify adding favorite."""
+        response = test_client.post(
+            f"/api/v1/photos/{test_photo.id}/favorite",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["is_favorite"] is True
+        
+        # Verify in database
+        from backend.db.models import PhotoFavorite
+        favorite = test_db_session.query(PhotoFavorite).filter(
+            PhotoFavorite.photo_id == test_photo.id,
+            PhotoFavorite.username == admin_user.username,
+        ).first()
+        assert favorite is not None
+
+    def test_toggle_favorite_remove(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        admin_user: "User",
+        test_db_session: "Session",
+    ):
+        """Verify removing favorite."""
+        from backend.db.models import PhotoFavorite
+        
+        # Add favorite first
+        favorite = PhotoFavorite(photo_id=test_photo.id, username=admin_user.username)
+        test_db_session.add(favorite)
+        test_db_session.commit()
+        
+        # Remove it
+        response = test_client.post(
+            f"/api/v1/photos/{test_photo.id}/favorite",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["is_favorite"] is False
+
+    def test_toggle_favorite_unauthenticated(
+        self,
+        test_client: TestClient,
+        test_photo: "Photo",
+    ):
+        """Verify 401 without auth."""
+        response = test_client.post(
+            f"/api/v1/photos/{test_photo.id}/favorite",
+        )
+        
+        assert response.status_code == 401
+
+    def test_toggle_favorite_photo_not_found(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 404 for non-existent photo."""
+        response = test_client.post(
+            "/api/v1/photos/99999/favorite",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 404
+
+    def test_bulk_add_favorites_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        test_photo_2: "Photo",
+    ):
+        """Verify bulk add operation."""
+        response = test_client.post(
+            "/api/v1/photos/favorites/bulk-add",
+            headers=auth_headers,
+            json={"photo_ids": [test_photo.id, test_photo_2.id]},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["added"] >= 0
+        assert data["already_favorites"] >= 0
+
+    def test_bulk_remove_favorites_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+        admin_user: "User",
+        test_db_session: "Session",
+    ):
+        """Verify bulk remove operation."""
+        from backend.db.models import PhotoFavorite
+        
+        # Add favorite first
+        favorite = PhotoFavorite(photo_id=test_photo.id, username=admin_user.username)
+        test_db_session.add(favorite)
+        test_db_session.commit()
+        
+        response = test_client.post(
+            "/api/v1/photos/favorites/bulk-remove",
+            headers=auth_headers,
+            json={"photo_ids": [test_photo.id]},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["removed"] >= 0
+
+
+class TestPhotoRetrieval:
+    """Test photo retrieval endpoints."""
+
+    def test_get_photo_by_id_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+    ):
+        """Verify photo retrieval by ID."""
+        response = test_client.get(
+            f"/api/v1/photos/{test_photo.id}",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == test_photo.id
+        assert data["filename"] == test_photo.filename
+
+    def test_get_photo_by_id_not_found(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 404 for non-existent photo."""
+        response = test_client.get(
+            "/api/v1/photos/99999",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 404
+
+
+class TestPhotoDeletion:
+    """Test photo deletion endpoints."""
+
+    def test_bulk_delete_photos_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_photo: "Photo",
+    ):
+        """Verify bulk delete (admin only)."""
+        response = test_client.post(
+            "/api/v1/photos/bulk-delete",
+            headers=auth_headers,
+            json={"photo_ids": [test_photo.id]},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "deleted" in data
+
+    def test_bulk_delete_photos_non_admin(
+        self,
+        test_client: TestClient,
+        regular_auth_headers: dict,
+        test_photo: "Photo",
+    ):
+        """Verify 403 for non-admin users."""
+        response = test_client.post(
+            "/api/v1/photos/bulk-delete",
+            headers=regular_auth_headers,
+            json={"photo_ids": [test_photo.id]},
+        )
+        
+        # Should be 403 or 401 depending on implementation
+        assert response.status_code in [403, 401]
+
+    def test_bulk_delete_photos_empty_list(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 400 with empty photo_ids."""
+        response = test_client.post(
+            "/api/v1/photos/bulk-delete",
+            headers=auth_headers,
+            json={"photo_ids": []},
+        )
+        
+        # May return 200 with 0 deleted or 400
+        assert response.status_code in [200, 400]
+
diff --git a/tests/test_api_tags.py b/tests/test_api_tags.py
new file mode 100644
index 0000000..8c93a9d
--- /dev/null
+++ b/tests/test_api_tags.py
@@ -0,0 +1,297 @@
+"""Medium priority tag API tests."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+from fastapi.testclient import TestClient
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+    from backend.db.models import Photo, Tag
+
+
+class TestTagListing:
+    """Test tag listing endpoints."""
+
+    def test_get_tags_success(
+        self,
+        test_client: TestClient,
+        test_db_session: "Session",
+    ):
+        """Verify tags list retrieval."""
+        from backend.db.models import Tag
+        
+        # Create a test tag
+        tag = Tag(tag="test-tag")
+        test_db_session.add(tag)
+        test_db_session.commit()
+        
+        response = test_client.get("/api/v1/tags")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        assert "total" in data
+
+    def test_get_tags_empty_list(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify empty list when no tags."""
+        response = test_client.get("/api/v1/tags")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        assert isinstance(data["items"], list)
+
+
+class TestTagCRUD:
+    """Test tag CRUD endpoints."""
+
+    def test_create_tag_success(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify tag creation."""
+        response = test_client.post(
+            "/api/v1/tags",
+            json={"tag_name": "new-tag"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["tag"] == "new-tag"
+        assert "id" in data
+
+    def test_create_tag_duplicate(
+        self,
+        test_client: TestClient,
+        test_db_session: "Session",
+    ):
+        """Verify returns existing tag if duplicate."""
+        from backend.db.models import Tag
+        
+        # Create tag first
+        tag = Tag(tag="duplicate-tag")
+        test_db_session.add(tag)
+        test_db_session.commit()
+        test_db_session.refresh(tag)
+        
+        # Try to create again
+        response = test_client.post(
+            "/api/v1/tags",
+            json={"tag_name": "duplicate-tag"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == tag.id
+        assert data["tag"] == "duplicate-tag"
+
+    def test_create_tag_strips_whitespace(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify whitespace handling."""
+        response = test_client.post(
+            "/api/v1/tags",
+            json={"tag_name": "  whitespace-tag  "},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        # Tag should be trimmed
+        assert "whitespace-tag" in data["tag"]
+
+    def test_update_tag_success(
+        self,
+        test_client: TestClient,
+        test_db_session: "Session",
+    ):
+        """Verify tag update."""
+        from backend.db.models import Tag
+        
+        tag = Tag(tag="old-name")
+        test_db_session.add(tag)
+        test_db_session.commit()
+        test_db_session.refresh(tag)
+        
+        response = test_client.put(
+            f"/api/v1/tags/{tag.id}",
+            json={"tag_name": "new-name"},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["tag"] == "new-name"
+
+    def test_update_tag_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 for non-existent tag."""
+        response = test_client.put(
+            "/api/v1/tags/99999",
+            json={"tag_name": "new-name"},
+        )
+        
+        assert response.status_code in [400, 404]  # Implementation dependent
+
+    def test_delete_tag_success(
+        self,
+        test_client: TestClient,
+        test_db_session: "Session",
+    ):
+        """Verify tag deletion."""
+        from backend.db.models import Tag
+        
+        tag = Tag(tag="delete-me")
+        test_db_session.add(tag)
+        test_db_session.commit()
+        test_db_session.refresh(tag)
+        
+        response = test_client.post(
+            "/api/v1/tags/delete",
+            json={"tag_ids": [tag.id]},
+        )
+        
+        assert response.status_code == 200
+
+    def test_delete_tag_not_found(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 404 handling."""
+        response = test_client.post(
+            "/api/v1/tags/delete",
+            json={"tag_ids": [99999]},
+        )
+        
+        # May return 200 with 0 deleted or error
+        assert response.status_code in [200, 400, 404]
+
+
+class TestPhotoTagOperations:
+    """Test photo-tag operations."""
+
+    def test_add_tags_to_photos_success(
+        self,
+        test_client: TestClient,
+        test_photo: "Photo",
+    ):
+        """Verify adding tags to photos."""
+        response = test_client.post(
+            "/api/v1/tags/photos/add",
+            json={
+                "photo_ids": [test_photo.id],
+                "tag_names": ["test-tag-1", "test-tag-2"],
+            },
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "photos_updated" in data
+        assert data["photos_updated"] >= 0
+
+    def test_add_tags_to_photos_empty_photo_ids(
+        self,
+        test_client: TestClient,
+    ):
+        """Verify 400 with empty photo_ids."""
+        response = test_client.post(
+            "/api/v1/tags/photos/add",
+            json={
+                "photo_ids": [],
+                "tag_names": ["test-tag"],
+            },
+        )
+        
+        assert response.status_code == 400
+
+    def test_add_tags_to_photos_empty_tag_names(
+        self,
+        test_client: TestClient,
+        test_photo: "Photo",
+    ):
+        """Verify 400 with empty tag_names."""
+        response = test_client.post(
+            "/api/v1/tags/photos/add",
+            json={
+                "photo_ids": [test_photo.id],
+                "tag_names": [],
+            },
+        )
+        
+        assert response.status_code == 400
+
+    def test_remove_tags_from_photos_success(
+        self,
+        test_client: TestClient,
+        test_photo: "Photo",
+        test_db_session: "Session",
+    ):
+        """Verify tag removal."""
+        from backend.db.models import Tag, PhotoTag
+        
+        # Add tag first
+        tag = Tag(tag="remove-me")
+        test_db_session.add(tag)
+        test_db_session.flush()
+        
+        photo_tag = PhotoTag(photo_id=test_photo.id, tag_id=tag.id)
+        test_db_session.add(photo_tag)
+        test_db_session.commit()
+        
+        # Remove it
+        response = test_client.post(
+            "/api/v1/tags/photos/remove",
+            json={
+                "photo_ids": [test_photo.id],
+                "tag_names": ["remove-me"],
+            },
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "tags_removed" in data
+
+    def test_get_photo_tags_success(
+        self,
+        test_client: TestClient,
+        test_photo: "Photo",
+        test_db_session: "Session",
+    ):
+        """Verify photo tags retrieval."""
+        from backend.db.models import Tag, PhotoTag
+        
+        tag = Tag(tag="photo-tag")
+        test_db_session.add(tag)
+        test_db_session.flush()
+        
+        photo_tag = PhotoTag(photo_id=test_photo.id, tag_id=tag.id)
+        test_db_session.add(photo_tag)
+        test_db_session.commit()
+        
+        response = test_client.get(f"/api/v1/tags/photos/{test_photo.id}")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "tags" in data
+        assert len(data["tags"]) > 0
+
+    def test_get_photo_tags_empty(
+        self,
+        test_client: TestClient,
+        test_photo: "Photo",
+    ):
+        """Verify empty list for untagged photo."""
+        response = test_client.get(f"/api/v1/tags/photos/{test_photo.id}")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "tags" in data
+        assert isinstance(data["tags"], list)
+
diff --git a/tests/test_api_users.py b/tests/test_api_users.py
new file mode 100644
index 0000000..de90de8
--- /dev/null
+++ b/tests/test_api_users.py
@@ -0,0 +1,273 @@
+"""High priority user API tests."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+from fastapi.testclient import TestClient
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+    from backend.db.models import User
+
+
+class TestUserListing:
+    """Test user listing endpoints."""
+
+    def test_list_users_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        admin_user: "User",
+    ):
+        """Verify users list (admin only)."""
+        response = test_client.get(
+            "/api/v1/users",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+        assert "total" in data
+
+    def test_list_users_non_admin(
+        self,
+        test_client: TestClient,
+        regular_auth_headers: dict,
+    ):
+        """Verify 403 for non-admin users."""
+        response = test_client.get(
+            "/api/v1/users",
+            headers=regular_auth_headers,
+        )
+        
+        assert response.status_code in [403, 401]
+
+    def test_list_users_with_pagination(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify pagination."""
+        response = test_client.get(
+            "/api/v1/users",
+            headers=auth_headers,
+            params={"page": 1, "page_size": 10},
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "items" in data
+
+
+class TestUserCRUD:
+    """Test user CRUD endpoints."""
+
+    def test_create_user_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify user creation (admin only)."""
+        response = test_client.post(
+            "/api/v1/users",
+            headers=auth_headers,
+            json={
+                "username": "newuser",
+                "email": "newuser@example.com",
+                "full_name": "New User",
+                "password": "password123",
+            },
+        )
+        
+        assert response.status_code == 201
+        data = response.json()
+        assert data["username"] == "newuser"
+        assert data["email"] == "newuser@example.com"
+
+    def test_create_user_duplicate_email(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        admin_user: "User",
+    ):
+        """Verify 400 with duplicate email."""
+        response = test_client.post(
+            "/api/v1/users",
+            headers=auth_headers,
+            json={
+                "username": "differentuser",
+                "email": admin_user.email,  # Duplicate email
+                "full_name": "Different User",
+                "password": "password123",
+            },
+        )
+        
+        assert response.status_code == 400
+
+    def test_create_user_duplicate_username(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        admin_user: "User",
+    ):
+        """Verify 400 with duplicate username."""
+        response = test_client.post(
+            "/api/v1/users",
+            headers=auth_headers,
+            json={
+                "username": admin_user.username,  # Duplicate username
+                "email": "different@example.com",
+                "full_name": "Different User",
+                "password": "password123",
+            },
+        )
+        
+        assert response.status_code == 400
+
+    def test_get_user_by_id_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        admin_user: "User",
+    ):
+        """Verify user retrieval."""
+        response = test_client.get(
+            f"/api/v1/users/{admin_user.id}",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == admin_user.id
+        assert data["username"] == admin_user.username
+
+    def test_get_user_by_id_not_found(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 404 for non-existent user."""
+        response = test_client.get(
+            "/api/v1/users/99999",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 404
+
+    def test_update_user_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        admin_user: "User",
+    ):
+        """Verify user update."""
+        response = test_client.put(
+            f"/api/v1/users/{admin_user.id}",
+            headers=auth_headers,
+            json={
+                "full_name": "Updated Name",
+            },
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["full_name"] == "Updated Name"
+
+    def test_delete_user_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        test_db_session: "Session",
+    ):
+        """Verify user deletion."""
+        from backend.db.models import User
+        from backend.utils.password import hash_password
+        from backend.constants.roles import DEFAULT_USER_ROLE
+        
+        # Create a user to delete
+        user = User(
+            username="deleteuser",
+            email="delete@example.com",
+            password_hash=hash_password("password"),
+            full_name="Delete User",
+            is_admin=False,
+            is_active=True,
+            role=DEFAULT_USER_ROLE,
+        )
+        test_db_session.add(user)
+        test_db_session.commit()
+        test_db_session.refresh(user)
+        
+        response = test_client.delete(
+            f"/api/v1/users/{user.id}",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+
+    def test_delete_user_non_admin(
+        self,
+        test_client: TestClient,
+        regular_auth_headers: dict,
+        admin_user: "User",
+    ):
+        """Verify 403 for non-admin."""
+        response = test_client.delete(
+            f"/api/v1/users/{admin_user.id}",
+            headers=regular_auth_headers,
+        )
+        
+        assert response.status_code in [403, 401]
+
+
+class TestUserActivation:
+    """Test user activation endpoints."""
+
+    def test_activate_user_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        inactive_user: "User",
+    ):
+        """Verify user activation."""
+        response = test_client.post(
+            f"/api/v1/users/{inactive_user.id}/activate",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["is_active"] is True
+
+    def test_deactivate_user_success(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+        regular_user: "User",
+    ):
+        """Verify user deactivation."""
+        response = test_client.post(
+            f"/api/v1/users/{regular_user.id}/deactivate",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["is_active"] is False
+
+    def test_activate_user_not_found(
+        self,
+        test_client: TestClient,
+        auth_headers: dict,
+    ):
+        """Verify 404 handling."""
+        response = test_client.post(
+            "/api/v1/users/99999/activate",
+            headers=auth_headers,
+        )
+        
+        assert response.status_code == 404
+