refactor(plex): playlists, playback, Discover and admin through the unit; the plex package is fenced

The last Plex HTTP files do no database work of their own, so ALLOWED is empty: no route builds
SQL, touches the session or a unit's table. Discover's rails move whole into plex/discover.py; a
user's playlists, watch-state writes (the race-safe finish, an early checkpoint, a status, a
season or show at once) and the watch-sync link go through plex/queries.py and plex/service.py.

The plex package joins FENCED: it reads no other unit's tables, says whose a row is through
userscope (the rollup and the watch sync's 8 inline filters moved to owned_by), and commits only in
its named jobs — the library sync, TMDB enrichment, the watch sync and its background pushes.
A playlist's has-item probe keeps the plain key lookup, so a NUL in it is still the 422 a test pins.

Pins 178 -> 0; basedpyright baseline 118 -> 106.

backend/app/discover/queries.py

@@ -6,7 +6,7 @@ Two rules shape this module:
 **Reuse the query layer, not the route.** The YouTube half calls `feedquery.filtered_query` /
 `sort_keys`, so the Home surface inherits the feed's own definitions of subscribed, blocked,
 explore-visible and in-progress for free, and cannot drift from them. The Plex half calls
-`plex.rollup` and the `plex.cards` serializers rather than `routes/plex/discover.py`: that route
+`plex.rollup` and the `plex.cards` serializers rather than `plex/discover.py`: that read-model
 narrows EVERY rail by the Plex sidebar's `LibraryFilters`, which is right for a surface that HAS a
 sidebar and wrong for a landing page that does not.
 
@@ -39,7 +39,7 @@ 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
+# (`plex/discover.py` imports it): the two surfaces should feel like the same app, and a rail
 # is a rail. One definition, because a comment was all that held two copies in step.
 RAIL_LIMIT = 20
 
@@ -221,7 +221,7 @@ def plex_in_progress(
 ) -> list[tuple[object, dict]]:
     """In-progress movies + partially-watched shows as (progressed_at, card).
 
-    Mirrors `routes/plex/discover.py::_continue_watching` — same `PlexState` predicate for movies (a
+    Mirrors `plex/discover.py::_continue_watching` — same `PlexState` predicate for movies (a
     NULL status still counts as started, so `notin_` alone would drop it), and the same shared
     `rollup.agg_subquery` + `apply_show_state_filter` for shows, so the two rails cannot disagree on
     what "in progress" means. What it does NOT inherit is that route's `LibraryFilters` narrowing:

backend/app/discover/rails.py

@@ -76,7 +76,7 @@ def surprise(
 ) -> list[dict]:
     """The hero's draw pool: unseen things that fit the budget, from both sources under ONE seed.
 
-    Interleaved rather than concatenated — the same alternation `routes/plex/discover.py` uses for its
+    Interleaved rather than concatenated — the same alternation `plex/discover.py` uses for its
     movie/show split — so a short pool never comes back all-YouTube or all-Plex just because one side
     happened to be queried first.