perf(admin): fix O(mangas x jobs) overview query that pinned Postgres (0.85.1)

The admin overview dashboard polls /admin/overview every 30s. Its
`manga_stats` aggregate evaluated MANGA_SYNC_STATE_CASE over the WHOLE
library (14.5k mangas) with no LIMIT, and that CASE runs a correlated
EXISTS over crawler_jobs. With no index on the `sync_chapter_list`
manga_id path, Postgres seqscanned all in-flight jobs *per manga* —
O(mangas x jobs). The disabled-analysis backlog (9.5k pending
`analyze_page` rows that can never match the sync-kind filter) inflated
every scan, so each call took 6-7 min. The 30s poll stacked ~10 of them
concurrently → load 11, Postgres at ~100%.

EXPLAIN ANALYZE on the live DB: the rewrite drops the query from 6-7 min
to ~1.5s (per-manga crawler_jobs seqscan → single index-backed pass).

Fixes:
- Rewrite `manga_stats` to collect the in-flight manga-id set in ONE pass
  over the sync jobs (index-backed), then classify via a hash semi-join —
  O(mangas + jobs), immune to the analyze_page backlog size.
- Add partial index crawler_jobs_sync_chapter_list_manga_idx (migration
  0029) covering the sync_chapter_list manga_id path; mirrors the existing
  sync_manga index (0020). Also speeds the per-row Mangas tab listing.
- statement_timeout backstop (5s) on the aggregate so a pathological plan
  can never pin a backend for minutes again.
- Single-flight + 10s TTL cache on the /admin/overview handler so
  concurrent pollers coalesce instead of stacking.
- Slow the dashboard poll 30s -> 60s (server-cached now anyway).

The in_progress rule in the rewrite is kept in lockstep with the first
arm of MANGA_SYNC_STATE_CASE (documented in both places).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.85.0",
+  "version": "0.85.1",
   "private": true,
   "type": "module",
   "scripts": {

frontend/src/routes/admin/+page.svelte

@@ -190,10 +190,13 @@
         openCrawlerStream();
         openAnalysisStream();
         sysTimer = setInterval(refreshSys, 5000);
+        // 60s: the overview aggregate is server-cached (~10s TTL) and these
+        // are background freshness updates, so a slower poll halves idle DB
+        // load without the dashboard feeling stale.
         overviewTimer = setInterval(() => {
             refreshOverview();
             refreshMetrics();
-        }, 30000);
+        }, 60000);
         if (typeof document !== 'undefined') {
             document.addEventListener('visibilitychange', onVisibilityChange);
             window.addEventListener('pagehide', onPageHide);