Standardize timezone usage to timezone.utc

- Replaced all UTC imports with timezone imports
- Changed all datetime.now(UTC) to datetime.now(timezone.utc)
- Consistent with booking.py and more explicit about timezone usage
- Updated models.py, routes/auth.py, and routes/invites.py

backend/routes/auth.py

@@ -1,5 +1,5 @@
 """Authentication routes for register, login, logout, and current user."""
-from datetime import datetime, UTC
+from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, Response, status
 from sqlalchemy import select
@@ -90,7 +90,7 @@ async def register(
     # Mark invite as spent
     invite.status = InviteStatus.SPENT
     invite.used_by_id = user.id
-    invite.spent_at = datetime.now(UTC)
+    invite.spent_at = datetime.now(timezone.utc)
 
     await db.commit()
     await db.refresh(user)

backend/routes/invites.py

@@ -1,5 +1,5 @@
 """Invite routes for public check, user invites, and admin management."""
-from datetime import datetime, UTC
+from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, status, Query
 from sqlalchemy import select, func, desc
@@ -228,7 +228,7 @@ async def revoke_invite(
         )
 
     invite.status = InviteStatus.REVOKED
-    invite.revoked_at = datetime.now(UTC)
+    invite.revoked_at = datetime.now(timezone.utc)
     await db.commit()
     await db.refresh(invite)