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/tests/test_playlist_move.py

@@ -280,13 +280,13 @@ def test_no_add_can_land_in_the_target_while_a_move_holds_it(db, make_user):
     from sqlalchemy import text
 
     from app.db import SessionLocal
-    from app.routes import playlists as plroutes
+    from app.sync.playlists import lock_playlists
 
     user = make_user()
     _seed_videos(db, ["v1"])
     src = _playlist(db, user, "Source", ["v1"])
     dst = _playlist(db, user, "Target", [])
-    plroutes.lock_playlists(db, src.id, dst.id)  # what the move holds until its commit
+    lock_playlists(db, src.id, dst.id)  # what the move holds until its commit
     with SessionLocal() as other:
         other.execute(text("SET LOCAL lock_timeout = '300ms'"))
         other.add(PlaylistItem(playlist_id=dst.id, video_id="v1", position=0))