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/tests/test_plex_discover.py

@@ -103,6 +103,16 @@ def test_discover_shape_and_recently_added_order(db, monkeypatch):
     assert genres.get("Drama") == 2 and genres.get("Comedy") == 1
 
 
+def test_genre_tiles_carry_poster_art(db, monkeypatch):
+    # Each genre tile carries up to 4 poster rating_keys for the 2×2 collage (top_art_keys): the top
+    # movies in that genre, best-rated then newest. No ratings here → newest-first; shows are excluded.
+    user = _seed(db)
+    d = _get(db, user, monkeypatch, "/api/plex/discover?scope=both").json()
+    art = {g["value"]: g["art"] for g in d["genres"]}
+    assert art["Drama"] == ["m1", "m3"]  # both Drama movies, m1 (newer) first
+    assert art["Comedy"] == ["m2"]
+
+
 def test_continue_watching_movies_and_shows(db, monkeypatch):
     user = _seed(db)
     d = _get(db, user, monkeypatch, "/api/plex/discover?scope=both").json()