feat(plex): redesign browse tiles — 2-poster montage top, readable label band

The full-bleed 4-poster collage drowned the on-tile text. Split PosterTile in two: the top
half shows up to 2 montage posters, the bottom half is a solid colour-hash band that carries
the label, so the text is always readable. Genre tiles drop the count (just the label now);
collection tiles show the name on one truncated line plus a sub-line of item count · total
runtime. A group with no art falls back to an all-colour tile.

Backend: `/collections` cards gain `total_runtime_s` (a new `runtime_by_key` grouped sum) and
their montage art is capped at 2. Frontend: PosterTile reworked; a `collectionMeta` helper
builds the count·runtime sub-line via the existing formatRuntime.

Verified live: collection and genre tiles render 2 posters over a readable colour band with
the right sub-lines; colour fallback intact for art-less groups.

Gate: tsc/lint/knip/prettier clean, 423 vitest; backend ruff clean, collections tests green
(+ a new art/runtime assertion).

frontend/src/components/PlexDiscover.tsx

@@ -204,9 +204,8 @@ export default function PlexDiscover({
                 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"
+                className="h-24"
               />
             ))}
           </div>