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).

frontend/src/components/PlexDiscover.dom.test.tsx

@@ -78,7 +78,7 @@ const fullPayload = {
   scope: "both",
   continue_watching: [card("cw1", "Resume Me")],
   recently_added: [card("ra1", "Just Added")],
-  genres: [{ value: "Drama", count: 12 }],
+  genres: [{ value: "Drama", count: 12, art: [] }],
   because_you_watched: { anchor: { id: "a1", title: "Anchor" }, items: [card("bw1", "Rec One")] },
 };