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/discover/queries.py

@@ -34,6 +34,7 @@ from app.reader.models import PlaylistItem, VideoState
 from app.plex import rollup
 from app.plex.cards import display_title, leaf_card, movie_card, show_card
 from app.plex.query import apply_show_state_filter, content_libs
+from app.userscope import owned_by
 
 # How many rows a single rail may carry — Home's rails and the Plex module's Discover rails alike
 # (`routes/plex/discover.py` imports it): the two surfaces should feel like the same app, and a rail
@@ -201,7 +202,7 @@ def _unseen_movie_conds(uid: int):
         .select_from(PlexState)
         .where(
             PlexState.item_id == PlexItem.id,
-            PlexState.user_id == uid,
+            owned_by(PlexState.user_id, uid),
             or_(
                 PlexState.status.in_(("watched", "hidden")),
                 # Started counts as "not a surprise" too — see the note in `yt_surprise`.
@@ -229,7 +230,9 @@ def plex_in_progress(
     if movie_libs:
         mq = (
             select(PlexItem, PlexState)
-            .join(PlexState, and_(PlexState.item_id == PlexItem.id, PlexState.user_id == uid))
+            .join(
+                PlexState, and_(PlexState.item_id == PlexItem.id, owned_by(PlexState.user_id, uid))
+            )
             .where(
                 PlexItem.kind == "movie",
                 PlexItem.library_id.in_(movie_libs),
@@ -323,7 +326,9 @@ def next_episode(db: Session, uid: int, show_id: int, *, lang: str) -> dict | No
     )
     base = (
         select(PlexItem, PlexState)
-        .outerjoin(PlexState, and_(PlexState.item_id == PlexItem.id, PlexState.user_id == uid))
+        .outerjoin(
+            PlexState, and_(PlexState.item_id == PlexItem.id, owned_by(PlexState.user_id, uid))
+        )
         .where(PlexItem.show_id == show_id, PlexItem.kind == "episode")
         .limit(1)
     )