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>

frontend/src/lib/api/admin.test.ts

@@ -24,7 +24,9 @@ import {
     updateCrawlerSession,
     clearCrawlerSessionExpired,
     listDeadJobs,
-    requeueDeadJobs
+    requeueDeadJobs,
+    listActiveJobs,
+    listMissingCovers
 } from './admin';
 
 function ok(body: unknown, status = 200): Response {
@@ -350,7 +352,19 @@ describe('admin crawler api client', () => {
     const statusFixture = {
         daemon: 'running',
         phase: { state: 'fetching_metadata', index: 3, total: 10, title: 'One Piece' },
-        workers: [{ state: 'idle' }],
+        worker_count: 2,
+        active_chapters: [
+            {
+                manga_id: 'm-1',
+                manga_title: 'Bleach',
+                chapter_id: 'c-1',
+                chapter_number: 12,
+                pages_done: 4,
+                pages_total: 20
+            }
+        ],
+        current_cover: { manga_id: 'm-2', manga_title: 'Naruto' },
+        covers_queued: 7,
         last_pass: { at: null, discovered: 0, upserted: 0, covers_fetched: 0, mangas_failed: 0 },
         session: { expired: false, configured: true },
         browser: 'healthy',
@@ -361,15 +375,38 @@ describe('admin crawler api client', () => {
         expect(crawlerStatusStreamUrl()).toMatch(/\/v1\/admin\/crawler\/stream$/);
     });
 
-    it('getCrawlerStatus GETs /v1/admin/crawler', async () => {
+    it('getCrawlerStatus GETs /v1/admin/crawler with live chapter/cover fields', async () => {
         fetchSpy.mockResolvedValueOnce(ok(statusFixture));
         const s = await getCrawlerStatus();
         expect(s.queue.dead).toBe(4);
         expect(s.phase?.state).toBe('fetching_metadata');
+        expect(s.active_chapters[0].pages_done).toBe(4);
+        expect(s.active_chapters[0].pages_total).toBe(20);
+        expect(s.current_cover?.manga_title).toBe('Naruto');
+        expect(s.covers_queued).toBe(7);
         const url = fetchSpy.mock.calls[0][0] as string;
         expect(url).toMatch(/\/v1\/admin\/crawler$/);
     });
 
+    it('listActiveJobs GETs /v1/admin/crawler/active-jobs with search', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok({ items: [], page: { limit: 20, offset: 0, total: 0 } })
+        );
+        await listActiveJobs({ search: 'bleach' });
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/crawler\/active-jobs\?/);
+        expect(url).toContain('search=bleach');
+    });
+
+    it('listMissingCovers GETs /v1/admin/crawler/covers', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok({ items: [{ manga_id: 'm-1', manga_title: 'X' }], page: { limit: 20, offset: 0, total: 1 } })
+        );
+        const r = await listMissingCovers();
+        expect(r.items[0].manga_title).toBe('X');
+        expect(fetchSpy.mock.calls[0][0]).toMatch(/\/v1\/admin\/crawler\/covers$/);
+    });
+
     it('runCrawlerPass POSTs /v1/admin/crawler/run', async () => {
         fetchSpy.mockResolvedValueOnce(ok({ started: true }));
         const r = await runCrawlerPass();