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/explore.py

@@ -15,7 +15,7 @@ from sqlalchemy.orm import Session
 
 from app import logs, sysconfig
 from app.models import Channel, ExploredChannel, SearchFind, Subscription, Video
-from app.reader.models import PlaylistItem, VideoState
+from app.reader import queries as reader_queries
 from app.sync.videos import insert_stubs, stub_from_playlist_item, apply_video_details
 from app.youtube.client import YouTubeClient, YouTubeError, is_playlist_gone
 
@@ -122,15 +122,12 @@ def purge_unkept_explored(db: Session) -> dict:
     if not orphan_ids:
         return {"expired": expired, "videos_deleted": 0, "channels_deleted": 0}
 
-    state_exists = select(VideoState.id).where(VideoState.video_id == Video.id).exists()
-    pl_exists = select(PlaylistItem.id).where(PlaylistItem.video_id == Video.id).exists()
     find_exists = select(SearchFind.id).where(SearchFind.video_id == Video.id).exists()
     videos_deleted = db.execute(
         delete(Video).where(
             Video.channel_id.in_(orphan_ids),
             Video.via_explore.is_(True),
-            ~state_exists,
-            ~pl_exists,
+            ~reader_queries.kept_by_a_reader(Video.id),
             ~find_exists,
         )
     ).rowcount
@@ -159,15 +156,12 @@ def purge_unkept_search(db: Session, grace_days: int | None = None) -> dict:
     0 quota."""
     grace = sysconfig.get_int(db, "search_grace_days") if grace_days is None else grace_days
     cutoff = datetime.now(timezone.utc) - timedelta(days=grace)
-    state_exists = select(VideoState.id).where(VideoState.video_id == Video.id).exists()
-    pl_exists = select(PlaylistItem.id).where(PlaylistItem.video_id == Video.id).exists()
     sub_exists = select(Subscription.id).where(Subscription.channel_id == Video.channel_id).exists()
     deleted = db.execute(
         delete(Video).where(
             Video.via_search.is_(True),
             Video.created_at < cutoff,
-            ~state_exists,
-            ~pl_exists,
+            ~reader_queries.kept_by_a_reader(Video.id),
             ~sub_exists,
         )
     ).rowcount