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/admin/+page.svelte

@@ -10,7 +10,8 @@
     rejectRequest,
     resolveQuarantine,
     deleteQuarantineFile,
-    deleteQuarantineViaLidarr
+    deleteQuarantineViaLidarr,
+    refetchMissingCovers
   } from '$lib/api/admin';
   import { qk } from '$lib/api/queries';
   import { copyForCode } from '$lib/api/error-copy';
@@ -176,6 +177,24 @@
       showToast(copyForCode((e as { code?: string }).code));
     }
   }
+
+  // ---- Cover art bulk refetch ----
+  let bulkBusy = $state(false);
+  let bulkResult = $state<string | null>(null);
+
+  async function onBulkRefetch() {
+    if (bulkBusy) return;
+    bulkBusy = true;
+    bulkResult = null;
+    try {
+      const { queued } = await refetchMissingCovers();
+      bulkResult = `Queued ${queued} albums for cover refetch.`;
+    } catch (e) {
+      bulkResult = `Failed: ${(e as { code?: string })?.code ?? 'unknown'}`;
+    } finally {
+      bulkBusy = false;
+    }
+  }
 </script>
 
 <svelte:head><title>{pageTitle('Admin · Overview')}</title></svelte:head>
@@ -257,6 +276,25 @@
     {/if}
   </section>
 
+  <!-- Cover art bulk refetch -->
+  <section class="rounded border border-border bg-surface p-4">
+    <h2 class="font-display text-lg font-medium">Cover art</h2>
+    <p class="mt-1 text-sm text-text-secondary">
+      Refetch cover art for albums that are missing one or where the previous attempt failed.
+    </p>
+    <button
+      type="button"
+      onclick={onBulkRefetch}
+      disabled={bulkBusy}
+      class="mt-3 rounded-md bg-accent px-4 py-2 text-sm font-medium text-text-primary disabled:opacity-50"
+    >
+      {bulkBusy ? 'Queueing…' : 'Refetch missing covers'}
+    </button>
+    {#if bulkResult}
+      <p class="mt-2 text-sm">{bulkResult}</p>
+    {/if}
+  </section>
+
   <!-- Quarantine preview -->
   <section class="space-y-3">
     <header class="flex items-baseline justify-between">