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/routes/albums/[id]/+page.svelte

@@ -2,6 +2,7 @@
   import { Play, Shuffle } from 'lucide-svelte';
   import { page } from '$app/state';
   import { createAlbumQuery } from '$lib/api/queries';
+  import { qk } from '$lib/api/queries';
   import { pageTitle } from '$lib/branding';
   import TrackRow from '$lib/components/TrackRow.svelte';
   import LibrarySkeleton from '$lib/components/LibrarySkeleton.svelte';
@@ -12,6 +13,9 @@
   import { FALLBACK_COVER } from '$lib/media/covers';
   import type { ApiError } from '$lib/api/client';
   import { playQueue } from '$lib/player/store.svelte';
+  import { user } from '$lib/auth/store.svelte';
+  import { refetchAlbumCover } from '$lib/api/admin';
+  import { useQueryClient } from '@tanstack/svelte-query';
 
   const id = $derived(page.params.id ?? '');
   const queryStore = $derived(createAlbumQuery(id));
@@ -58,6 +62,27 @@
       isStartingPlay = false;
     }
   }
+
+  const queryClient = useQueryClient();
+  let refetching = $state(false);
+  let refetchError = $state<string | null>(null);
+
+  const isAdmin = $derived(user.value?.is_admin === true);
+
+  async function onRefetchCover() {
+    const data = query.data;
+    if (!data || refetching) return;
+    refetching = true;
+    refetchError = null;
+    try {
+      await refetchAlbumCover(data.id);
+      await queryClient.invalidateQueries({ queryKey: qk.album(data.id) });
+    } catch (e) {
+      refetchError = (e as { code?: string })?.code ?? 'refetch failed';
+    } finally {
+      refetching = false;
+    }
+  }
 </script>
 
 <svelte:head>
@@ -127,6 +152,21 @@
             <Shuffle size={16} strokeWidth={1.5} />
             Shuffle
           </button>
+          {#if isAdmin}
+            <button
+              type="button"
+              onclick={onRefetchCover}
+              disabled={refetching}
+              aria-label={`Refetch cover for ${album.title}`}
+              title={album.cover_art_source === 'none' ? 'No cover found previously — try again' : 'Refetch from MusicBrainz'}
+              class="flex items-center gap-2 rounded-md border border-border bg-surface px-3 py-2 text-sm font-medium text-text-secondary hover:border-accent disabled:opacity-50"
+            >
+              Refetch cover
+            </button>
+          {/if}
+          {#if refetchError}
+            <span class="text-sm text-oxblood">{refetchError}</span>
+          {/if}
         </div>
       </div>
     </header>