fix(plex): show-library collections get montage art too (A3)

The collection browse-tile montage queried only movie members, so show-library
collections (IMDb Popular, TMDb Trending, … in the TV library) rendered as bare colour
tiles. Union the art from PlexShow members as well (a collection lives in one library, so
exactly one side has art); fall back to the collection's own poster, then the colour hash.
Runtime stays movie-only (shows have no single duration).

Verified live: every collection now shows a montage (0 art-less tiles, was ~all the show
collections).

Gate: backend ruff clean, 5 collections tests (+ a show-collection art assertion).

backend/app/routes/plex/collections.py

@@ -11,6 +11,7 @@ from app.models import (
     PlexCollection,
     PlexItem,
     PlexLibrary,
+    PlexShow,
     User,
 )
 from app.plex import sync as plex_sync
@@ -95,17 +96,25 @@ def collections(
             func.lower(PlexCollection.title),
         )
     ).scalars().all()
-    # A montage of the top films per tile + the collection's total runtime (color hash + no-runtime is
-    # the client fallback for show-only / empty collections). Two grouped queries over the movie libs.
-    movie_ids, _show_ids = _content_libs(db)
+    # A montage of the top titles per tile + the collection's total runtime. Art unions MOVIE and SHOW
+    # members (a collection lives in one library, so exactly one side has art) — a show collection would
+    # otherwise render blank; the collection's own poster is the next fallback, then the colour hash.
+    # Runtime is movie-only (shows have no single duration).
+    movie_ids, show_ids = _content_libs(db)
     keys = [c.rating_key for c in rows]
-    art = top_art_keys(db, PlexItem, movie_ids, PlexItem.collection_keys, keys, per=2)
+    art_m = top_art_keys(db, PlexItem, movie_ids, PlexItem.collection_keys, keys, per=2)
+    art_s = top_art_keys(db, PlexShow, show_ids, PlexShow.collection_keys, keys, per=2)
     runtime = runtime_by_key(db, PlexItem, movie_ids, PlexItem.collection_keys, keys)
+
+    def _art(c: PlexCollection) -> list[str]:
+        member = art_m.get(c.rating_key) or art_s.get(c.rating_key)
+        return member or ([c.rating_key] if c.thumb_key else [])
+
     return {
         "collections": [
             {
                 **_collection_card(c),
-                "art": art.get(c.rating_key, []),
+                "art": _art(c),
                 "total_runtime_s": runtime.get(c.rating_key, 0),
             }
             for c in rows