feat: cover retry backfill + admin force-resync for manga & chapter (0.50.0)

Adds a per-tick cover-backfill pass to the crawler daemon so mangas whose
cover download failed on first attempt get retried — the metadata pass's
early-stop optimisation otherwise prevents the walk from revisiting them.

Adds admin-only POST /admin/mangas/:id/resync and POST /admin/chapters/:id/resync
that refetch metadata + cover (or chapter content with force_refetch) from the
crawler source synchronously and return the refreshed row. Surfaced in the
UI as "Force resync" buttons on the manga detail and reader pages,
admin-only via session.user.is_admin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -14,7 +14,9 @@ import {
     createAdminUser,
     listAdminMangas,
     listAdminChapters,
-    getSystemStats
+    getSystemStats,
+    resyncManga,
+    resyncChapter
 } from './admin';
 
 function ok(body: unknown, status = 200): Response {
@@ -242,4 +244,88 @@ describe('admin api client', () => {
         const s = await getSystemStats();
         expect(s.disk).toBeNull();
     });
+
+    // ---- force resync ----
+
+    it('resyncManga POSTs to /v1/admin/mangas/{id}/resync and returns the envelope', async () => {
+        const resp = {
+            manga: {
+                id: 'm-1',
+                title: 'T',
+                status: 'ongoing',
+                alt_titles: [],
+                description: null,
+                cover_image_path: 'mangas/m-1/cover.jpg',
+                created_at: '2026-01-01T00:00:00Z',
+                updated_at: '2026-01-02T00:00:00Z',
+                authors: [],
+                genres: [],
+                tags: []
+            },
+            metadata_status: 'updated',
+            cover_fetched: true
+        };
+        fetchSpy.mockResolvedValueOnce(ok(resp));
+        const got = await resyncManga('m-1');
+        expect(got.metadata_status).toBe('updated');
+        expect(got.cover_fetched).toBe(true);
+        expect(got.manga.id).toBe('m-1');
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/mangas\/m-1\/resync$/);
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.method).toBe('POST');
+    });
+
+    it('resyncManga surfaces 503 service_unavailable when the daemon is off', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            envelope(503, 'service_unavailable', 'crawler daemon is disabled')
+        );
+        await expect(resyncManga('m-1')).rejects.toMatchObject({
+            status: 503,
+            code: 'service_unavailable'
+        });
+    });
+
+    it('resyncChapter POSTs to /v1/admin/chapters/{id}/resync and returns the envelope', async () => {
+        const resp = {
+            chapter: {
+                id: 'c-1',
+                manga_id: 'm-1',
+                number: 1,
+                title: 'Foo',
+                page_count: 7,
+                created_at: '2026-01-01T00:00:00Z'
+            },
+            outcome: 'fetched',
+            pages: 7
+        };
+        fetchSpy.mockResolvedValueOnce(ok(resp));
+        const got = await resyncChapter('c-1');
+        expect(got.outcome).toBe('fetched');
+        expect(got.pages).toBe(7);
+        expect(got.chapter.page_count).toBe(7);
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/chapters\/c-1\/resync$/);
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.method).toBe('POST');
+    });
+
+    it('resyncChapter handles the "skipped" outcome envelope', async () => {
+        const resp = {
+            chapter: {
+                id: 'c-1',
+                manga_id: 'm-1',
+                number: 1,
+                title: null,
+                page_count: 7,
+                created_at: '2026-01-01T00:00:00Z'
+            },
+            outcome: 'skipped',
+            pages: null
+        };
+        fetchSpy.mockResolvedValueOnce(ok(resp));
+        const got = await resyncChapter('c-1');
+        expect(got.outcome).toBe('skipped');
+        expect(got.pages).toBeNull();
+    });
 });