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/routes/library/[id]/+page.svelte

@@ -211,14 +211,9 @@
         const repo = auth.getRepository();
         const seasons = $libraryItems.filter((i) => i.kind === "season");
 
-        const [episodes, current] = await Promise.all([
-          repo.getSeriesEpisodes(itemId),
-          // Best-effort: a series still renders if the anchor cannot be resolved.
-          repo.getSeriesCurrentEpisode(itemId).catch((e) => {
-            log.warn("Could not resolve the current episode:", e);
-            return null;
-          }),
-        ]);
+        // One call, one season fan-out: asking for episodes and the current
+        // episode separately walked every season twice (DR-295).
+        const { episodes, current } = await repo.getSeriesView(itemId);
 
         seasonData = groupEpisodesBySeason(seasons, episodes);
         currentEpisode = current;