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/feed.py

@@ -13,7 +13,6 @@ from sqlalchemy.orm.exc import StaleDataError
 from app import quota
 from app.auth import current_user
 from app.db import get_db
-from app.utils import iso
 from app.models import (
     LIVE_OR_UPCOMING,
     BlockedChannel,
@@ -30,6 +29,7 @@ from app.models import (
     VideoState,
 )
 from app.sync.videos import parse_iso8601_duration
+from app.utils import iso
 from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api", tags=["feed"])