feat(crawler): live cover + chapter-content observability with realtime page counts

Extends the live dashboard so an operator can see exactly what's being
fetched, in realtime:

- Chapters being crawled now are tracked in the status as `active_chapters`
  (manga title · ch.N) with a live page counter that climbs per stored page
  (set_chapter_pages, pushed via the existing watch→SSE). The dispatcher
  registers each via an RAII ChapterGuard (sync Mutex) that removes the
  entry on completion, panic, or timeout-drop — replacing the old per-worker
  slot model.
- Covers: status now carries the cover being fetched now (`current_cover`,
  set around download_and_store_cover in both the metadata pass and backfill)
  and a `covers_queued` backlog count; CoverBackfill phase gains index/total.
- Two paginated backlog endpoints (fetched on demand, auto-refreshed when the
  live counts change): GET /admin/crawler/active-jobs (which chapters of which
  mangas are queued/running) and GET /admin/crawler/covers (mangas missing a
  cover). repo: list_active_jobs, list_missing_cover_mangas, count_missing_covers.
- dispatch_target now also returns manga title + chapter number.

Frontend: the crawler page replaces the Workers table with an Active-chapters
table (live page bars), adds a current-cover line + covers-queued figure, and
two backlog sections (Queued chapters / Queued covers) with search + Pager,
auto-refetched via $effect on the live counts.

Tests: status guard/page + cover unit tests; repo list/count tests; endpoint
tests; frontend api tests. Version 0.53.1 -> 0.54.0.

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

backend/tests/repo_chapter.rs

@@ -109,7 +109,7 @@ async fn dispatch_target_prefers_most_recent_live_source(pool: PgPool) {
         seed_chapter_with_two_live_sources(&pool).await;
 
     let row = dispatch_target(&pool, chapter_id).await.unwrap();
-    let (_manga_id, source_url) =
+    let (_manga_id, source_url, _title, _number) =
         row.expect("two live sources should yield a dispatch target");
     assert_eq!(
         source_url, new_url,
@@ -133,7 +133,7 @@ async fn dispatch_target_skips_dropped_sources(pool: PgPool) {
     .unwrap();
 
     let row = dispatch_target(&pool, chapter_id).await.unwrap();
-    let (_manga_id, source_url) =
+    let (_manga_id, source_url, _title, _number) =
         row.expect("a single live source should still yield a dispatch target");
     assert!(
         source_url != new_url,