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/api_admin_crawler.rs

@@ -64,6 +64,102 @@ async fn seed_dead_job(pool: &PgPool, title: &str) -> Uuid {
     job_id
 }
 
+/// Seed a chapter-content job in a given state ('pending'/'running').
+async fn seed_job(pool: &PgPool, title: &str, state: &str) {
+    let manga_id = Uuid::new_v4();
+    let chapter_id = Uuid::new_v4();
+    sqlx::query("INSERT INTO mangas (id, title) VALUES ($1, $2)")
+        .bind(manga_id)
+        .bind(title)
+        .execute(pool)
+        .await
+        .unwrap();
+    sqlx::query("INSERT INTO chapters (id, manga_id, number) VALUES ($1, $2, 1)")
+        .bind(chapter_id)
+        .bind(manga_id)
+        .execute(pool)
+        .await
+        .unwrap();
+    sqlx::query("INSERT INTO crawler_jobs (id, payload, state) VALUES ($1, $2, $3)")
+        .bind(Uuid::new_v4())
+        .bind(json!({
+            "kind": "sync_chapter_content",
+            "source_id": "target",
+            "chapter_id": chapter_id,
+            "source_chapter_key": "k",
+        }))
+        .bind(state)
+        .execute(pool)
+        .await
+        .unwrap();
+}
+
+/// Seed a manga with no cover + a live source row (queued for cover fetch).
+async fn seed_missing_cover(pool: &PgPool, title: &str) {
+    let manga_id = Uuid::new_v4();
+    sqlx::query("INSERT INTO mangas (id, title, cover_image_path) VALUES ($1, $2, NULL)")
+        .bind(manga_id)
+        .bind(title)
+        .execute(pool)
+        .await
+        .unwrap();
+    sqlx::query("INSERT INTO sources (id, name, base_url) VALUES ('target','T','http://x') ON CONFLICT DO NOTHING")
+        .execute(pool)
+        .await
+        .unwrap();
+    sqlx::query(
+        "INSERT INTO manga_sources (source_id, source_manga_key, manga_id, source_url) \
+         VALUES ('target', $1, $2, 'http://x/m')",
+    )
+    .bind(format!("k-{manga_id}"))
+    .bind(manga_id)
+    .execute(pool)
+    .await
+    .unwrap();
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn active_jobs_and_covers_lists_over_http(pool: PgPool) {
+    seed_job(&pool, "Naruto", "pending").await;
+    seed_job(&pool, "Bleach", "running").await;
+    seed_missing_cover(&pool, "One Piece").await;
+    let h = harness(pool.clone());
+    let cookie = seed_admin(&pool, &h.app).await;
+
+    // Queued/active chapters.
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get_with_cookie("/api/v1/admin/crawler/active-jobs", &cookie))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = body_json(resp).await;
+    assert_eq!(body["page"]["total"], 2);
+
+    // Queued covers.
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get_with_cookie("/api/v1/admin/crawler/covers", &cookie))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = body_json(resp).await;
+    assert_eq!(body["page"]["total"], 1);
+    assert_eq!(body["items"][0]["manga_title"], "One Piece");
+
+    // Both are admin-gated.
+    let (_u, plain) = register_user(&h.app).await;
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get_with_cookie("/api/v1/admin/crawler/active-jobs", &plain))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::FORBIDDEN);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn get_status_requires_admin(pool: PgPool) {
     let h = harness(pool);