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/admin.test.ts

@@ -42,6 +42,7 @@ vi.mock('$lib/api/admin', async () => {
     deleteQuarantineViaLidarr: vi.fn().mockResolvedValue({}),
     triggerScan: vi.fn().mockResolvedValue({}),
     refetchMissingCovers: vi.fn().mockResolvedValue({ queued: 0 }),
+    researchMissingArt: vi.fn().mockResolvedValue({ version: 1 }),
     createScanScheduleQuery: vi.fn(() =>
       readable({
         data: {
@@ -111,7 +112,8 @@ import {
   rejectRequest,
   resolveQuarantine,
   deleteQuarantineFile,
-  deleteQuarantineViaLidarr
+  deleteQuarantineViaLidarr,
+  researchMissingArt
 } from '$lib/api/admin';
 import OverviewPage from './+page.svelte';
 
@@ -264,6 +266,15 @@ describe('admin Overview page', () => {
     expect(select.value).toBe('off');
   });
 
+  test('renders Re-search missing art button and invokes on click', async () => {
+    setupOverview();
+    render(OverviewPage);
+    const btn = screen.getByRole('button', { name: /re-search missing art/i });
+    expect(btn).toBeInTheDocument();
+    await fireEvent.click(btn);
+    expect(researchMissingArt).toHaveBeenCalled();
+  });
+
   test('Library scan card shows Processing badge when library tally is populated and in-flight', async () => {
     const { readable } = await import('svelte/store');
     (createScanStatusQuery as ReturnType<typeof vi.fn>).mockReturnValueOnce(