Replace deprecated datetime.utcnow with shared naive-UTC helper

All 29 call sites now use app.timeutil.utcnow(), which returns naive
UTC via the non-deprecated API. Semantics are unchanged on purpose:
every datetime column is TIMESTAMP WITHOUT TIME ZONE, so going aware
here would poison naive/aware comparisons. The TIMESTAMPTZ migration
(Phase 2) now has a single place to change. (BE-01)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/api/v1/endpoints/settings.py

@@ -5,6 +5,7 @@ from sqlmodel import Session, select
 
 from app.auth import get_current_user
 from app.db import get_session
+from app.timeutil import utcnow
 from app.models.models import UserSettings, UserSettingsRead, UserSettingsUpdate
 
 router = APIRouter(prefix="/settings", tags=["settings"])
@@ -52,7 +53,7 @@ def update_settings(
         settings = UserSettings(key="default", data=body.data)
     else:
         settings.data = body.data
-    settings.updated_at = datetime.utcnow()
+    settings.updated_at = utcnow()
     session.add(settings)
     session.commit()
     session.refresh(settings)