fix: resolve flake8 C901/F401 errors in main.py, gmail_client.py, test_mail_sources.py

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/38868887-87b6-4032-8713-e90e56ae3318

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/tests/test_mail_sources.py

@@ -489,10 +489,6 @@ class TestGmailAPIMailSource:
         )
         source_id = create_resp.json()["id"]
 
-        # Inject tokens directly into DB via the DB session
-        from sqlalchemy.orm import Session
-        from app.models.mail_source import MailSource as MS
-
         # Use the authed_client's DB override — patch the ORM object instead
         mock_service = MagicMock()
         mock_service.users.return_value.getProfile.return_value.execute.return_value = {