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).

backend/app/routes/plex/collections.py

@@ -16,7 +16,7 @@ from app.models import (
 from app.plex import sync as plex_sync
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
 from app.plex.lookups import by_rating_key
-from app.plex.query import like_escape, top_art_keys
+from app.plex.query import like_escape, runtime_by_key, top_art_keys
 from app.routes.plex._shared import _content_libs
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
@@ -95,11 +95,22 @@ def collections(
             func.lower(PlexCollection.title),
         )
     ).scalars().all()
-    # A 2×2 poster collage per tile: the top movies in each collection (color hash is the client
-    # fallback for show-only / empty collections). One windowed query across the content movie libs.
+    # A montage of the top films per tile + the collection's total runtime (color hash + no-runtime is
+    # the client fallback for show-only / empty collections). Two grouped queries over the movie libs.
     movie_ids, _show_ids = _content_libs(db)
-    art = top_art_keys(db, PlexItem, movie_ids, PlexItem.collection_keys, [c.rating_key for c in rows])
-    return {"collections": [{**_collection_card(c), "art": art.get(c.rating_key, [])} for c in rows]}
+    keys = [c.rating_key for c in rows]
+    art = top_art_keys(db, PlexItem, movie_ids, PlexItem.collection_keys, keys, per=2)
+    runtime = runtime_by_key(db, PlexItem, movie_ids, PlexItem.collection_keys, keys)
+    return {
+        "collections": [
+            {
+                **_collection_card(c),
+                "art": art.get(c.rating_key, []),
+                "total_runtime_s": runtime.get(c.rating_key, 0),
+            }
+            for c in rows
+        ]
+    }
 
 
 def _editable_collection_or_403(db: Session, rating_key: str) -> PlexCollection: