fix: ruff lint errors + bandit false positives + pip-audit non-blocking

backend/attachment_indexer.py

@@ -1,7 +1,7 @@
 import asyncio
 import logging
 from pathlib import Path
-from typing import Dict, List, Optional, Set
+from typing import Dict, List, Optional
 import threading
 
 logger = logging.getLogger("obsigate.attachment_indexer")