Add comprehensive tests for SQLite facade, metadata extractors, and security helpers

- Introduced unit tests for SQLite facade helpers, including schema validation, query building, and async operations.
- Added integration tests for basic SQLite queries and transaction handling.
- Implemented additional tests for metadata extraction functions, focusing on dimension handling, EXIF data, and video metadata.
- Created tests for security core helpers, covering environment variable checks, IP validation, and CSRF protection.
- Included compatibility tests for route exports to ensure proper functionality.

.github/workflows/python-tests.yml

@@ -21,7 +21,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r requirements.txt
-          pip install xenon
+          pip install xenon radon flake8 flake8-cognitive-complexity
 
       - name: Check Python files encoding (no UTF-8 BOM)
         run: python scripts/check_no_bom.py mjr_am_backend mjr_am_shared tests __init__.py scripts
@@ -29,6 +29,9 @@ jobs:
       - name: Complexity gate (stage A)
         run: python -m xenon -b F -m D -a C mjr_am_backend mjr_am_shared tests __init__.py
 
+      - name: Complexity gate (new files, CC <= 10)
+        run: python scripts/check_cc_changed.py
+
   test:
     runs-on: ubuntu-22.04
     strategy: