refactor(reader): the HTTP layer reads through queries and writes through the service

Feed, playlists, saved views, tags and the Home pins do no database work in their routes any more:
reads go through reader/queries (and feedquery for a feed page, its counts and facets), writes
through reader/service and discover/pins, which flush; the handlers commit. Reader is fenced below
its HTTP layer too: its owner filters go through userscope, and the catalog's jobs ask Reader
instead of joining its tables (the auto-tagger's system tags, the cleanup jobs' keep rule, the
maintenance notice). A playlist sync's catalog ingest moves to sync/videos.

backend/app/sync/maintenance.py

@@ -28,7 +28,7 @@ from sqlalchemy.orm import Session
 from app import logs, progress, quota, sysconfig
 from app.config import settings
 from app.models import Video
-from app.reader.models import Playlist, PlaylistItem
+from app.reader import queries as reader_queries
 from app.notifications import create_notification
 from app.sync.runner import get_service_user
 from app.sync.videos import apply_video_details, parse_dt
@@ -89,11 +89,7 @@ def _collect_user_impact(db: Session, videos: list[Video]) -> dict[int, list[dic
     # Anyone who had it in a playlist — which now also covers "saved": Watch later became a
     # built-in playlist, so "saved" is no longer a VideoState status (the old status=="saved"
     # scan matched nothing and under-counted impacted users).
-    for user_id, video_id in db.execute(
-        select(Playlist.user_id, PlaylistItem.video_id)
-        .join(PlaylistItem, PlaylistItem.playlist_id == Playlist.id)
-        .where(PlaylistItem.video_id.in_(ids))
-    ):
+    for user_id, video_id in reader_queries.playlisted_by(db, ids):
         impact.setdefault(user_id, set()).add(video_id)
     return {
         user_id: [