Fix CI failures: add backend/conftest.py for module resolution and run black formatting

- Add backend/conftest.py that inserts the backend directory into sys.path,
  fixing ModuleNotFoundError when pytest runs from the backend/ directory
  (as CI does with `cd backend && pytest tests/`)
- Run black formatter on all 28 backend files that needed reformatting
- All 53 tests pass with both `pytest tests/` and `python -m pytest tests/`

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/beb47db2-da25-416a-8fb0-c1452a3b22a7

backend/conftest.py

@@ -0,0 +1,6 @@
+import sys
+from pathlib import Path
+
+# Ensure the backend directory is on sys.path so that `app` is importable
+# when pytest is invoked from the backend/ directory (e.g., `cd backend && pytest tests/`).
+sys.path.insert(0, str(Path(__file__).resolve().parent))