refactor(discover): one shelf query per kind, one RAIL_LIMIT, a true rollup comment

The comfort shelf resolved each pinned channel with three queries (the
row, its unwatched count, its newest video) and each playlist with two.
channels_summary gets the count and the newest row per channel as window
functions over one filtered set, playlists_summary reads every pinned list
in one query, and the rows come in one IN() each — so the shelf's cost no
longer grows with its length. A test counts the statements.

RAIL_LIMIT had two definitions held in step by a comment; the Plex Discover
route now imports the domain's. And rollup's "continue the latest-started
episode" was false: it takes the last in-progress one in broadcast order,
which is also what Home's next_episode picks.

backend/app/routes/plex/discover.py

@@ -32,12 +32,12 @@ from app.plex.query import (
     meta_filter_conds,
     top_art_keys,
 )
+from app.discover.queries import RAIL_LIMIT
 from app.plex.tmdb import resolve_recs
 from app.routes.plex._shared import _norm_lang
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
-RAIL_LIMIT = 20
 GENRE_TILES = 12
 SURPRISE_PER_KIND = 3  # Surprise-me: at most this many movies + this many shows (6 total)