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/utils/tempo.py

@@ -0,0 +1,34 @@
+"""Time helpers: the DB holds naive UTC, presentation converts to display_timezone (#42)."""
+from datetime import datetime, timezone
+from zoneinfo import ZoneInfo
+
+from src.backend.config import settings
+
+
+def utcnow() -> datetime:
+    """Naive UTC now (the whole DB moves to naive UTC, #42)."""
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
+def as_utc_naive(dt: datetime | None) -> datetime | None:
+    """Aware -> UTC naive; naive is taken as UTC already; None stays None."""
+    if dt is None or dt.tzinfo is None:
+        return dt
+    return dt.astimezone(timezone.utc).replace(tzinfo=None)
+
+
+def to_utc_aware(dt: datetime) -> datetime:
+    """Naive (UTC) or aware datetime -> aware UTC."""
+    if dt.tzinfo is None:
+        return dt.replace(tzinfo=timezone.utc)
+    return dt.astimezone(timezone.utc)
+
+
+def iso_utc(dt: datetime | None) -> str:
+    """ISO 8601 with explicit +00:00; "" for None (CSV cells)."""
+    return to_utc_aware(dt).isoformat() if dt else ""
+
+
+def to_local(dt: datetime) -> datetime:
+    """Naive UTC (or aware) datetime -> aware datetime in settings.display_timezone."""
+    return to_utc_aware(dt).astimezone(ZoneInfo(settings.display_timezone))