fix: add missing timezone import in main.py

_list_backup_files uses datetime.fromtimestamp(ts, tz=timezone.utc)
but only datetime was imported, not timezone

backend/main.py

@@ -11,7 +11,7 @@ import string
 import time
 from concurrent.futures import ThreadPoolExecutor
 from contextlib import asynccontextmanager
-from datetime import datetime
+from datetime import datetime, timezone
 from functools import partial
 from pathlib import Path
 from typing import Optional, List, Dict, Any