fix: replace datetime.UTC with timezone.utc for Python 3.10 compatibility

datetime.UTC was introduced in Python 3.11. Users on 3.10 get an ImportError.
Also lower ruff/mypy target-version from py313 to py310 to match minimum supported version.

backend/app/db/models.py

@@ -1,5 +1,5 @@
 import uuid
-from datetime import UTC, datetime
+from datetime import datetime, timezone
 from typing import Any
 
 from sqlalchemy import JSON, Boolean, DateTime, Float, ForeignKey, Integer, String, Text
@@ -9,7 +9,7 @@ from app.db.database import Base
 
 
 def _now() -> datetime:
-    return datetime.now(UTC)
+    return datetime.now(timezone.utc)
 
 
 def _uuid() -> str: