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/api/v1/api.py

@@ -1,17 +1,34 @@
 """
 API v1 router aggregation.
 """
+
 from fastapi import APIRouter
 
-from app.api.v1.endpoints import auth, users, mail_accounts, notifications, subscriptions, admin, providers
+from app.api.v1.endpoints import (
+    auth,
+    users,
+    mail_accounts,
+    notifications,
+    subscriptions,
+    admin,
+    providers,
+)
 
 api_router = APIRouter()
 
 # Include all endpoint routers
 api_router.include_router(auth.router, prefix="/auth", tags=["Authentication"])
 api_router.include_router(users.router, prefix="/users", tags=["Users"])
-api_router.include_router(mail_accounts.router, prefix="/mail-accounts", tags=["Mail Accounts"])
-api_router.include_router(providers.router, prefix="/providers", tags=["Providers & Gmail"])
-api_router.include_router(notifications.router, prefix="/notifications", tags=["Notifications"])
-api_router.include_router(subscriptions.router, prefix="/subscriptions", tags=["Subscriptions"])
+api_router.include_router(
+    mail_accounts.router, prefix="/mail-accounts", tags=["Mail Accounts"]
+)
+api_router.include_router(
+    providers.router, prefix="/providers", tags=["Providers & Gmail"]
+)
+api_router.include_router(
+    notifications.router, prefix="/notifications", tags=["Notifications"]
+)
+api_router.include_router(
+    subscriptions.router, prefix="/subscriptions", tags=["Subscriptions"]
+)
 api_router.include_router(admin.router, prefix="/admin", tags=["Admin"])