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>

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.85.0"
+version = "0.85.1"
 dependencies = [
  "anyhow",
  "argon2",