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/app/workers/celery_app.py

@@ -1,6 +1,7 @@
 """
 Celery application for background email processing tasks.
 """
+
 from celery import Celery
 from celery.schedules import crontab
 import logging
@@ -14,7 +15,7 @@ celery_app = Celery(
     "pop3_forwarder",
     broker=settings.CELERY_BROKER_URL,
     backend=settings.CELERY_RESULT_BACKEND,
-    include=["app.workers.tasks"]
+    include=["app.workers.tasks"],
 )
 
 # Celery configuration