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.ts

@@ -161,3 +161,23 @@ export function createQuarantineActionsQuery(limit: number = 50) {
     staleTime: 60_000
   });
 }
+
+// Admin cover art ---------------------------------------------------------
+
+export type RefetchAlbumCoverResponse = {
+  album_id: string;
+  cover_art_path: string | null;
+  cover_art_source: string | null;
+};
+
+export async function refetchAlbumCover(albumId: string): Promise<RefetchAlbumCoverResponse> {
+  return api.post<RefetchAlbumCoverResponse>(`/api/admin/albums/${albumId}/cover/refetch`, {});
+}
+
+export type RefetchMissingResponse = {
+  queued: number;
+};
+
+export async function refetchMissingCovers(): Promise<RefetchMissingResponse> {
+  return api.post<RefetchMissingResponse>('/api/admin/covers/refetch-missing', {});
+}