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/tests/unit/test_gmail_service.py

@@ -1,6 +1,7 @@
 """
 Unit tests for Gmail service module.
 """
+
 import pytest
 from unittest.mock import MagicMock, patch, AsyncMock
 from app.services.gmail_service import GmailService, GmailInjectionError, GMAIL_SCOPES
@@ -91,7 +92,9 @@ class TestGmailService:
         service = GmailService(access_token="test-access-token")
 
         mock_api = MagicMock()
-        mock_api.users().messages().insert().execute.side_effect = Exception("API Error")
+        mock_api.users().messages().insert().execute.side_effect = Exception(
+            "API Error"
+        )
         service._service = mock_api
 
         with pytest.raises(GmailInjectionError, match="Failed to inject email"):