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")] },
 };
 

frontend/src/components/PlexDiscover.tsx

@@ -4,7 +4,7 @@ import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { RotateCcw } from "lucide-react";
 import { qk } from "../lib/queryKeys";
 import { api, type PlexCard, type PlexDiscover as PlexDiscoverData } from "../lib/api";
-import { hueOf, hueTileGradient } from "../lib/hueTile";
+import PosterTile from "./PosterTile";
 import { plexLang } from "../i18n";
 import { toggleWatchState } from "../lib/plexWatch";
 import { usePlex } from "./PlexProvider";
@@ -199,21 +199,16 @@ export default function PlexDiscover({
         <section className="mb-5">
           <h2 className="mb-3 text-sm font-semibold">{t("plex.discover.byGenre")}</h2>
           <div className="grid grid-cols-2 gap-3 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-6">
-            {d.genres.map((g) => {
-              const hue = hueOf(g.value);
-              return (
-                <button
-                  key={g.value}
-                  type="button"
-                  onClick={() => onGenre(g.value)}
-                  className="glass-hover relative flex h-20 items-end overflow-hidden rounded-xl p-3 text-left transition"
-                  style={{ background: hueTileGradient(hue) }}
-                >
-                  <span className="text-sm font-semibold text-white drop-shadow">{g.value}</span>
-                  <span className="absolute right-2 top-2 text-2xs text-white/70">{g.count}</span>
-                </button>
-              );
-            })}
+            {d.genres.map((g) => (
+              <PosterTile
+                key={g.value}
+                title={g.value}
+                art={g.art}
+                topRight={<span className="text-2xs text-white/80 drop-shadow">{g.count}</span>}
+                onClick={() => onGenre(g.value)}
+                className="h-20"
+              />
+            ))}
           </div>
         </section>
       )}

frontend/src/components/PosterTile.tsx

@@ -0,0 +1,57 @@
+import { type ReactNode } from "react";
+import { hueOf, hueTileGradient } from "../lib/hueTile";
+
+// A browse tile: a 2×2 collage of the group's top poster art (served from /api/plex/image/{key}) under
+// a dark scrim with the label, falling back to a color hash when the group has no art — so a tile never
+// renders blank. Shared by the Discover genre tiles and the Playlist/Collection browse tiles.
+export default function PosterTile({
+  title,
+  subtitle,
+  topRight,
+  art,
+  onClick,
+  className,
+}: {
+  title: string;
+  subtitle?: ReactNode; // a bottom sub-line under the title (e.g. "12 items")
+  topRight?: ReactNode; // top-right corner content (a count, or an icon)
+  art?: string[]; // up to 4 poster rating_keys; fewer/none falls back to the color hash
+  onClick: () => void;
+  className?: string; // caller sets the height (e.g. "h-20" / "h-28")
+}) {
+  const posters = (art ?? []).slice(0, 4);
+  return (
+    <button
+      type="button"
+      onClick={onClick}
+      title={title}
+      className={`glass-hover group relative flex flex-col justify-end overflow-hidden rounded-xl text-left transition ${className ?? ""}`}
+      style={posters.length === 0 ? { background: hueTileGradient(hueOf(title)) } : undefined}
+    >
+      {posters.length > 0 && (
+        <div className="absolute inset-0 grid grid-cols-2 grid-rows-2">
+          {Array.from({ length: 4 }).map((_, i) =>
+            posters[i] ? (
+              <img
+                key={i}
+                src={`/api/plex/image/${posters[i]}`}
+                alt=""
+                loading="lazy"
+                className="h-full w-full object-cover"
+              />
+            ) : (
+              <div key={i} style={{ background: hueTileGradient(hueOf(title + i)) }} />
+            ),
+          )}
+        </div>
+      )}
+      <div className="absolute inset-0 bg-gradient-to-t from-black/75 via-black/10 to-transparent" />
+      {/* After the scrim in DOM order → paints above it without a z-index token. */}
+      {topRight != null && <span className="absolute right-2 top-2">{topRight}</span>}
+      <div className="relative p-3">
+        <div className="line-clamp-2 text-sm font-semibold text-white drop-shadow">{title}</div>
+        {subtitle != null && <div className="text-2xs text-white/80">{subtitle}</div>}
+      </div>
+    </button>
+  );
+}

frontend/src/components/plexBrowseLists.tsx

@@ -1,45 +1,19 @@
-import { useState, type ReactNode } from "react";
+import { useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
 import { Layers, ListMusic } from "lucide-react";
 import { qk } from "../lib/queryKeys";
 import { api } from "../lib/api";
-import { hueOf, hueTileGradient } from "../lib/hueTile";
 import { useDebounced } from "../lib/useDebounced";
 import { LoadingState, StateMessage } from "./QueryState";
+import PosterTile from "./PosterTile";
 
 // The Playlist / Collection browse surfaces reached from the header pill (Piece 2): a grid of tiles,
 // each a way INTO that area — a playlist tile opens the playlist view, a collection tile filters the
 // title grid by that collection. Both reuse the existing list endpoints (plexPlaylists / plexCollections
-// union), so this is a pure front-end surface. Tiles are coloured by a title hash (no per-item art to
-// fetch/proxy), matching the Discover genre tiles.
-
-function Tile({
-  title,
-  count,
-  icon,
-  onClick,
-}: {
-  title: string;
-  count: ReactNode;
-  icon: ReactNode;
-  onClick: () => void;
-}) {
-  const hue = hueOf(title);
-  return (
-    <button
-      type="button"
-      onClick={onClick}
-      title={title}
-      className="glass-hover group relative flex h-28 flex-col justify-end overflow-hidden rounded-xl p-3 text-left transition"
-      style={{ background: hueTileGradient(hue) }}
-    >
-      <span className="absolute right-2 top-2 text-white/70">{icon}</span>
-      <span className="line-clamp-2 text-sm font-semibold text-white drop-shadow">{title}</span>
-      <span className="text-2xs text-white/70">{count}</span>
-    </button>
-  );
-}
+// union), so this is a pure front-end surface. Collection tiles show a poster collage of their top
+// films (PosterTile), falling back to a title-hash colour; playlists have no server art yet, so they
+// take the colour fallback.
 
 const GRID = "grid grid-cols-2 gap-3 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-5 xl:grid-cols-6";
 
@@ -63,12 +37,13 @@ export function PlexPlaylistsBrowse({ onOpen }: { onOpen: (id: number) => void }
   return (
     <div className={GRID}>
       {playlists.map((p) => (
-        <Tile
+        <PosterTile
           key={p.id}
           title={p.title}
-          count={t("plex.browse.items", { count: p.item_count })}
-          icon={<ListMusic className="h-4 w-4" />}
+          subtitle={t("plex.browse.items", { count: p.item_count })}
+          topRight={<ListMusic className="h-4 w-4 text-white/80 drop-shadow" />}
           onClick={() => onOpen(p.id)}
+          className="h-28"
         />
       ))}
     </div>
@@ -108,12 +83,16 @@ export function PlexCollectionsBrowse({ onPick }: { onPick: (id: string, title:
       ) : (
         <div className={GRID}>
           {collections.map((c) => (
-            <Tile
+            <PosterTile
               key={c.id}
               title={c.title}
-              count={c.child_count != null ? t("plex.browse.items", { count: c.child_count }) : ""}
-              icon={<Layers className="h-4 w-4" />}
+              art={c.art}
+              subtitle={
+                c.child_count != null ? t("plex.browse.items", { count: c.child_count }) : undefined
+              }
+              topRight={<Layers className="h-4 w-4 text-white/80 drop-shadow" />}
               onClick={() => onPick(c.id, c.title)}
+              className="h-28"
             />
           ))}
         </div>