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/lib/api/plex.ts

@@ -184,7 +184,8 @@ export interface PlexCollection {
   source?: string;
   editable: boolean;
   can_edit?: boolean; // effective: admin-marked editable AND a plain (non-smart/non-auto) collection
-  art?: string[]; // up to 4 poster rating_keys for the browse-tile collage (browse union only)
+  art?: string[]; // up to 2 poster rating_keys for the browse-tile montage (browse union only)
+  total_runtime_s?: number; // total runtime of all items, for the tile sub-line (browse union only)
 }
 export interface PlexPlaylist {
   id: number;