fix: import datetime class in User.save() — NameError blocked all registration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/user.py

@@ -117,7 +117,7 @@ class User:
         return result["credits_balance"] if result else 0
 
     async def save(self):
-        from datetime import timezone
+        from datetime import datetime, timezone
         db = await get_db()
         now = datetime.now(timezone.utc)
         user_data = {