feat(web/m7-353): admin cover-refetch UI (per-album + bulk)

Adds cover_art_source to AlbumRef, two admin API helpers (refetchAlbumCover,
refetchMissingCovers), a per-album retry button gated on is_admin in the album
detail page, and a bulk-refetch section in the admin overview.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/lib/api/admin.covers.test.ts

@@ -0,0 +1,33 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+vi.mock('./client', () => ({
+  api: {
+    get: vi.fn(),
+    post: vi.fn(),
+    put: vi.fn(),
+    del: vi.fn()
+  }
+}));
+
+import { refetchAlbumCover, refetchMissingCovers } from './admin';
+import { api } from './client';
+
+describe('admin covers API', () => {
+  beforeEach(() => vi.clearAllMocks());
+
+  it('refetchAlbumCover POSTs to the correct path', async () => {
+    (api.post as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      album_id: 'a1', cover_art_path: '/x.jpg', cover_art_source: 'mbcaa'
+    });
+    const got = await refetchAlbumCover('a1');
+    expect(api.post).toHaveBeenCalledWith('/api/admin/albums/a1/cover/refetch', {});
+    expect(got.cover_art_source).toBe('mbcaa');
+  });
+
+  it('refetchMissingCovers POSTs to the bulk endpoint', async () => {
+    (api.post as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({ queued: 7 });
+    const got = await refetchMissingCovers();
+    expect(api.post).toHaveBeenCalledWith('/api/admin/covers/refetch-missing', {});
+    expect(got.queued).toBe(7);
+  });
+});