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/lib.rs

@@ -238,6 +238,7 @@ use commands::{
     repository_get_resume_movies,
     repository_get_series_current_episode,
     repository_get_series_episodes,
+    repository_get_series_view,
     repository_get_similar_items,
     repository_get_stream_selection,
     repository_get_subtitle_url,
@@ -1014,6 +1015,7 @@ fn specta_builder() -> Builder<tauri::Wry> {
             repository_get_next_up_episodes,
             repository_get_series_episodes,
             repository_get_series_current_episode,
+            repository_get_series_view,
             repository_clear_watch_history,
             repository_get_recently_played_audio,
             repository_get_resume_movies,