refactor(feed): extract read-model to app.feedquery + admin_user sweep

B-core-3 (2026-08-17 review): decouple two things from router modules.

1. Feed read-model → new app/feedquery.py: the pure query-building and
   row-serialization layer (feed_columns, serialize, _filtered_query,
   to_tsquery_str, and the keyset-pagination helpers) moves out of
   routes/feed.py, which keeps only the route handlers, _feed_params, and
   the request models. The three external consumers (playlists, search,
   plex/browse) now import from app.feedquery instead of reaching into the
   feed router.

2. admin_user sweep: 7 modules imported admin_user via app.routes.admin
   (an incidental re-export of app.auth.admin_user). Repoint them all to
   the definition in app.auth (audit, channels, config, quota, scheduler,
   plex/admin, plex/browse); routes.admin keeps its own import for its use.

Pure code motion — no behaviour change. Full DB-lane suite 287 passed /
2 skipped; ruff clean.

backend/app/routes/playlists.py

@@ -9,8 +9,8 @@ from sqlalchemy.orm import Session, aliased
 from app import quota
 from app.auth import current_user, has_read_scope, has_write_scope, require_write_scope
 from app.db import get_db
+from app.feedquery import feed_columns, serialize
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
-from app.routes.feed import serialize, feed_columns
 from app.sync.playlists import (
     plan_push,
     push_playlist,