Backend: UTC nel DB, +00:00 nelle API, ora italiana solo in presentazione (#42)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/backend/services/auth_service.py

@@ -1,7 +1,8 @@
 """Authentication service - password hashing, API key management."""
 import hashlib
 import secrets
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timedelta
+from src.backend.utils.tempo import utcnow
 
 import bcrypt
 from fastapi import HTTPException, status
@@ -78,11 +79,6 @@ async def authorise_supervisor(db: AsyncSession, username: str, password: str) -
     return supervisor
 
 
-def utcnow() -> datetime:
-    """Naive UTC now (the whole DB moves to naive UTC, #42)."""
-    return datetime.now(timezone.utc).replace(tzinfo=None)
-
-
 def hash_api_key(key: str) -> str:
     """SHA-256 hex digest of an API key; this is all the DB ever stores."""
     return hashlib.sha256(key.encode("utf-8")).hexdigest()