feat(server/web/coverart): manual Re-search missing art button

POST /api/admin/cover-sources/research bumps cover_art_sources_meta.
current_version unconditionally; the next enrichment pass re-eligibles
every 'none' row. Existing positively-sourced rows are not affected —
the version-mismatch eligibility rule only kicks in for 'none'.

Admin Cover Art panel gains a "Re-search missing art" button that
calls the endpoint and shows a confirmation toast. Operator's escape
hatch when:

- They've added a Last.fm key and want to retry failed rows
  immediately rather than waiting for the next scheduled scan.
- An upstream provider's catalog has updated (e.g. Deezer added a
  back-catalog import).
- They want to verify a fix end-to-end before the next scheduled
  scan fires.

SettingsService.BumpVersion (the unconditional version of T5's
BumpVersionIfProvidersChanged) is the single-call DB writer; both
endpoints route through it.

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

web/src/routes/admin/+page.svelte

@@ -19,6 +19,7 @@
     deleteQuarantineViaLidarr,
     refetchMissingCovers,
     triggerScan,
+    researchMissingArt,
     type ScanSchedule,
     type ScanScheduleMode
   } from '$lib/api/admin';
@@ -315,6 +316,21 @@
   let bulkBusy = $state(false);
   let bulkResult = $state<string | null>(null);
 
+  // ---- Re-search missing art ----
+  let researchSaving = $state(false);
+
+  async function onResearchMissingArt() {
+    researchSaving = true;
+    try {
+      await researchMissingArt();
+      showToast('All previously-failed art will be re-attempted on the next scan.');
+    } catch (e) {
+      showToast(`Re-search failed: ${(e as { code?: string })?.code ?? 'unknown'}`);
+    } finally {
+      researchSaving = false;
+    }
+  }
+
   async function onBulkRefetch() {
     if (bulkBusy) return;
     bulkBusy = true;
@@ -603,6 +619,14 @@
       >
         {bulkBusy ? 'Queueing…' : 'Refetch missing covers'}
       </button>
+      <button
+        type="button"
+        onclick={onResearchMissingArt}
+        disabled={researchSaving}
+        class="rounded-md border border-border px-3 py-1.5 text-sm text-text-secondary hover:bg-surface-hover hover:text-text-primary disabled:opacity-50"
+      >
+        {researchSaving ? 'Bumping…' : 'Re-search missing art'}
+      </button>
 
       {#if coverage}
         {#if coverage.total === 0}