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/vision.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for the vision execution response."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 
 from pydantic import BaseModel, ConfigDict
 
@@ -28,4 +28,4 @@ class VisionReferenceImageResponse(BaseModel):
     note: str | None = None
     station_id: int | None = None
     device_code: str | None = None
-    acquired_at: datetime
+    acquired_at: UtcDateTime