feat(plex): poster collages on the genre + collection browse tiles

The Browse-by-genre tiles and the Collections browse tiles were flat color-hash
gradients. Give each a 2×2 collage of its top films' poster art, with the color hash
kept as the fallback so a group with no matching art (a show-only or empty collection)
never renders blank.

Backend: a shared `top_art_keys` windowed query returns, per key (a genre value or a
collection rating_key), the top few items' rating_keys — best-rated then newest, in one
query. Wired into the Discover genre tiles and the collections union endpoint; the client
renders the posters from the existing /api/plex/image/{key} endpoint, so no new image
plumbing. Frontend: a shared PosterTile component (collage + scrim + label, color-hash
fallback) replaces the two ad-hoc colored-tile implementations.

Verified live on localdev: genre tiles and collection tiles render real poster collages,
with the color fallback correctly kicking in for groups without art.

Gate: tsc/lint/knip/prettier clean, 423 vitest; backend ruff clean, 11 discover tests
(incl. a new genre-art assertion).

backend/app/routes/plex/discover.py

@@ -29,6 +29,7 @@ from app.plex.query import (
     apply_show_state_filter,
     genres_col,
     meta_filter_conds,
+    top_art_keys,
 )
 from app.plex.tmdb import resolve_recs
 from app.routes.plex._shared import _content_libs, _norm_lang
@@ -199,7 +200,16 @@ def _genre_tiles(
         ).all():
             counts[g] = counts.get(g, 0) + c
     top = sorted(counts.items(), key=lambda kv: (-kv[1], kv[0]))[:GENRE_TILES]
-    return [{"value": g, "count": c} for g, c in top]
+    # A 2×2 poster collage per tile: the top movies in that genre (color hash is the client fallback).
+    art = top_art_keys(
+        db,
+        PlexItem,
+        movie_ids,
+        genres_col(PlexItem, lang),
+        [g for g, _ in top],
+        extra_conds=(PlexItem.kind == "movie", *meta_filter_conds(PlexItem, p, lang)),
+    )
+    return [{"value": g, "count": c, "art": art.get(g, [])} for g, c in top]
 
 
 def _because_you_watched(