feat: web video transcoding, admin playback, and viewer fixes

Add on-demand H.264/AAC web playback (RQ, ffmpeg) with API routes and
Next.js proxies; extend admin UI with WebPlaybackVideo and shared hooks.
Store transcode cache beside pending-photos (WEB_VIDEO_CACHE_DIR / UPLOAD_DIR)
and ignore data/web_videos. Centralize FastAPI URL helpers, optional Vite
and Next base paths for subfolder deploy, and fix modal reopen by using
router.replace when closing the home photo viewer. Include migration,
install scripts, deployment doc updates, and CI admin build env tweak.

Made-with: Cursor

backend/services/tasks.py

@@ -345,3 +345,78 @@ def process_faces_task(
     finally:
         db.close()
 
+
+def transcode_video_for_web_task(photo_id: int) -> dict:
+    """Background job: build browser-safe MP4 (or mark original as ready if H.264/AAC)."""
+    import os
+    import traceback
+
+    from backend.db.models import Photo
+    from backend.db.session import SessionLocal
+    from backend.services.web_video_service import (
+        derived_mp4_path,
+        expire_web_playable_if_stale,
+        probe_browser_safe_video,
+        run_ffmpeg_web_transcode,
+        web_playback_is_ready,
+    )
+
+    db = SessionLocal()
+    try:
+        photo = (
+            db.query(Photo)
+            .filter(Photo.id == photo_id)
+            .with_for_update()
+            .first()
+        )
+        if not photo or photo.media_type != "video":
+            return {"ok": False, "error": "not_found"}
+
+        expire_web_playable_if_stale(photo)
+        if web_playback_is_ready(photo):
+            db.commit()
+            return {"ok": True, "skipped": True}
+
+        photo.web_transcode_status = "running"
+        photo.web_transcode_error = None
+        db.commit()
+
+        if not os.path.isfile(photo.path):
+            raise RuntimeError("Source video file not found on disk")
+
+        if probe_browser_safe_video(photo.path):
+            photo.web_playable_path = photo.path
+            photo.web_transcode_status = "ready"
+            photo.web_transcode_error = None
+            db.commit()
+            return {"ok": True, "native": True}
+
+        dst = derived_mp4_path(photo_id)
+        run_ffmpeg_web_transcode(photo.path, dst)
+        photo.web_playable_path = str(dst)
+        photo.web_transcode_status = "ready"
+        photo.web_transcode_error = None
+        db.commit()
+        return {"ok": True, "transcoded": True}
+    except Exception as e:
+        try:
+            db.rollback()
+        except Exception:
+            pass
+        try:
+            print(f"[Task] web transcode failed for photo {photo_id}: {e}")
+            traceback.print_exc()
+        except (BrokenPipeError, OSError):
+            pass
+        try:
+            photo = db.query(Photo).filter(Photo.id == photo_id).first()
+            if photo:
+                photo.web_transcode_status = "failed"
+                photo.web_transcode_error = str(e)[:2000]
+                db.commit()
+        except Exception:
+            pass
+        return {"ok": False, "error": str(e)}
+    finally:
+        db.close()
+