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/models/api/user.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for User operations."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 from typing import Optional
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -48,8 +48,8 @@ class UserResponse(BaseModel):
     language_pref: str
     theme_pref: str
     active: bool
-    created_at: datetime
-    last_login: Optional[datetime] = None
+    created_at: UtcDateTime
+    last_login: Optional[UtcDateTime] = None
 
 
 class UserPasswordChange(BaseModel):