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/endpoints/admin.py

@@ -1,4 +1,5 @@
 """Admin endpoints"""
+
 from fastapi import APIRouter, Depends
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select, func
@@ -13,24 +14,24 @@ router = APIRouter()
 @router.get("/stats")
 async def get_admin_stats(
     current_user: User = Depends(get_current_superuser),
-    db: AsyncSession = Depends(get_db)
+    db: AsyncSession = Depends(get_db),
 ):
     """Get overall system statistics (admin only)"""
-    
+
     # Count users
     user_count = await db.execute(select(func.count(User.id)))
     total_users = user_count.scalar()
-    
+
     # Count accounts
     account_count = await db.execute(select(func.count(MailAccount.id)))
     total_accounts = account_count.scalar()
-    
+
     # Count processing runs
     run_count = await db.execute(select(func.count(ProcessingRun.id)))
     total_runs = run_count.scalar()
-    
+
     return {
         "total_users": total_users,
         "total_mail_accounts": total_accounts,
-        "total_processing_runs": total_runs
+        "total_processing_runs": total_runs,
     }