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-tauri/src/commands/repository.rs

@@ -514,6 +514,24 @@ pub async fn repository_get_series_current_episode(
         .map_err(|e| format!("{:?}", e))
 }
 
+/// A series' episodes and the viewer's current episode, from one season
+/// fan-out. The series page used to ask for these as two commands, each of
+/// which walked every season.
+///
+/// TRACES: UR-062 | DR-101, DR-295
+#[tauri::command]
+#[specta::specta]
+pub async fn repository_get_series_view(
+    manager: State<'_, RepositoryManagerWrapper>,
+    handle: String,
+    series_id: String,
+) -> Result<series_progress::SeriesView, String> {
+    let repo = manager.0.get(&handle).ok_or("Repository not found")?;
+    series_progress::resolve_series_view(repo.as_ref(), &series_id)
+        .await
+        .map_err(|e| format!("{:?}", e))
+}
+
 /// Erase the viewer's watch history for an item.
 ///
 /// Clears the played flag and the resume position; on a series or season the