style(api): complete the S1a import-order fix (R6 S1a follow-up)

Round-1 review finding #4 named only audit.py/plex.py, but the same misplaced
`from app.utils import iso` sat in channels/downloads/feed/notifications too — the
round missed them (ruff enforces no import order here, so the gate was silent).
Move them to alphabetical position for consistency. No behaviour change.

backend/app/routes/notifications.py

@@ -11,8 +11,8 @@ from sqlalchemy.orm import Session
 from app.auth import current_user
 from app.db import get_db
 from app.models import Notification, User
-from app.utils import iso
 from app.notifications import trim_read
+from app.utils import iso
 
 router = APIRouter(prefix="/api/me/notifications", tags=["notifications"])