Fix flake8 C901 complexity violations and F824 unused global in 5 files

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/e58d59de-a79d-48e1-a4fc-ffb61b868593

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

backend/app/main.py

@@ -148,7 +148,6 @@ def create_app() -> FastAPI:
     @app.on_event("shutdown")
     async def shutdown_event():
         """Clean up background tasks on application shutdown"""
-        global background_task
         if background_task:
             logger.info("Cancelling IMAP polling background task")
             background_task.cancel()