chore: Update sensitive information in documentation and code to use environment variables

This commit replaces hardcoded sensitive information, such as database passwords and secret keys, in the README and deployment documentation with placeholders and instructions to use environment variables. This change enhances security by preventing exposure of sensitive data in the codebase. Additionally, it updates the database session management to raise an error if the DATABASE_URL environment variable is not set, ensuring proper configuration for development environments.

tests/README.md

@@ -0,0 +1,111 @@
+# Running Backend API Tests
+
+## Quick Start
+
+### Option 1: Using the test runner script (Recommended)
+```bash
+./run_tests.sh
+```
+
+### Option 2: Using npm script
+```bash
+npm run test:backend
+```
+
+### Option 3: Manual command
+```bash
+export PYTHONPATH=$(pwd)
+export SKIP_DEEPFACE_IN_TESTS=1
+./venv/bin/python3 -m pytest tests/ -v
+```
+
+## Where to See Test Results
+
+**Test results are displayed in your terminal/console** where you run the command.
+
+### Example Output
+
+When tests run successfully, you'll see output like:
+
+```
+tests/test_api_auth.py::TestLogin::test_login_success_with_valid_credentials PASSED
+tests/test_api_auth.py::TestLogin::test_login_failure_with_invalid_credentials PASSED
+tests/test_api_auth.py::TestTokenRefresh::test_refresh_token_success PASSED
+...
+========================= 26 passed in 2.34s =========================
+```
+
+### Understanding the Output
+
+- **PASSED** (green) - Test passed successfully
+- **FAILED** (red) - Test failed (shows error details)
+- **ERROR** (red) - Test had an error during setup/teardown
+- **SKIPPED** (yellow) - Test was skipped
+
+### Verbose Output
+
+The `-v` flag shows:
+- Each test function name
+- Pass/fail status for each test
+- Summary at the end
+
+### Detailed Failure Information
+
+If a test fails, pytest shows:
+- The test that failed
+- The assertion that failed
+- The actual vs expected values
+- A traceback showing where the error occurred
+
+## Test Coverage
+
+To see coverage report:
+```bash
+export PYTHONPATH=$(pwd)
+export SKIP_DEEPFACE_IN_TESTS=1
+./venv/bin/python3 -m pytest tests/ --cov=backend --cov-report=term-missing
+```
+
+This shows:
+- Which lines of code are covered by tests
+- Which lines are missing coverage
+- Overall coverage percentage
+
+## Running Specific Tests
+
+### Run a single test file
+```bash
+./venv/bin/python3 -m pytest tests/test_api_auth.py -v
+```
+
+### Run a specific test class
+```bash
+./venv/bin/python3 -m pytest tests/test_api_auth.py::TestLogin -v
+```
+
+### Run a specific test
+```bash
+./venv/bin/python3 -m pytest tests/test_api_auth.py::TestLogin::test_login_success_with_valid_credentials -v
+```
+
+## CI/CD Test Results
+
+In CI (GitHub Actions/Gitea Actions), test results appear in:
+1. **CI Logs** - Check the "Run backend tests" step in the workflow
+2. **Test Artifacts** - JUnit XML files are generated for test reporting tools
+3. **Coverage Reports** - Coverage XML files are generated
+
+## Troubleshooting
+
+### Tests not showing output?
+- Make sure you're running in a terminal (not an IDE output panel that might hide output)
+- Try adding `-s` flag: `pytest tests/ -v -s` (shows print statements)
+
+### Tests hanging?
+- Check if database is accessible
+- Verify `SKIP_DEEPFACE_IN_TESTS=1` is set (prevents DeepFace from loading)
+
+### Import errors?
+- Make sure virtual environment is activated or use `./venv/bin/python3`
+- Verify all dependencies are installed: `./venv/bin/pip install -r requirements.txt`
+