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/channels.py

@@ -11,7 +11,6 @@ from sqlalchemy.orm import Session
 from app import quota, sysconfig
 from app.auth import current_user, require_human, require_write_scope
 from app.db import get_db
-from app.utils import iso
 from app.models import (
     LIVE_OR_UPCOMING,
     BlockedChannel,
@@ -29,6 +28,7 @@ from app.routes.admin import admin_user
 from app.sync.explore import explore_ingest_page
 from app.sync.runner import run_recent_backfill
 from app.sync.subscriptions import apply_channel_details
+from app.utils import iso
 from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api/channels", tags=["channels"])