perf(series): list a series' episodes with one concurrent season walk

"More info" on Frasier took ~10 s. The series page asked Rust for the
episodes and for the current episode as two commands; each walked every
season, and each walk fetched the eleven seasons one after another. So the
wait was the sum of twenty-two listings, each a cache read queued behind
whatever the database was writing — measured at ~4 s per walk on a Fairphone
5 while the launch-time catalog sync ran.

Seasons are now fetched together (gather_season_episodes), so a walk waits
for its slowest season, not the sum. And repository_get_series_view returns
the episodes and the current episode from one walk, with Next Up and resume
fetched alongside it; the series page makes that one call.

Under today's single database connection the cache reads themselves still
queue on its mutex; the concurrency pays off fully once reads get their own
connections. Halving the walks helps regardless.

Test first: ten 100 ms seasons took 1.01 s sequentially; now well under the
400 ms bound, with a failing season still leaving the rest.

DR-295, UT-264.

src/lib/api/repository-client.ts

@@ -3,7 +3,13 @@
 // NO direct HTTP calls - everything routes through Rust backend
 
 import { commands } from "./bindings";
-import type { DownloadDiskUsage, JRayActor, SearchScope, StreamSelection } from "./bindings";
+import type {
+  DownloadDiskUsage,
+  JRayActor,
+  SearchScope,
+  SeriesView,
+  StreamSelection,
+} from "./bindings";
 import type { QualityPreset } from "./quality-presets";
 import type {
   Library,
@@ -164,6 +170,17 @@ export class RepositoryClient {
     return commands.repositoryGetSeriesCurrentEpisode(this.ensureHandle(), seriesId);
   }
 
+  /**
+   * A series' episodes and the episode the viewer is up to, from one season
+   * fan-out in Rust. Prefer this over calling `getSeriesEpisodes` and
+   * `getSeriesCurrentEpisode` together — each of those walks every season.
+   *
+   * TRACES: UR-062 | DR-101, DR-295
+   */
+  async getSeriesView(seriesId: string): Promise<SeriesView> {
+    return commands.repositoryGetSeriesView(this.ensureHandle(), seriesId);
+  }
+
   /**
    * Erase watch history for an item. On a series or season the server applies
    * it to everything inside, so the container returns to "never watched".