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/lib/api/admin.ts

@@ -320,6 +320,17 @@ export async function testCoverProvider(id: string): Promise<TestCoverProviderRe
   return api.post<TestCoverProviderResponse>(`/api/admin/cover-sources/${id}/test`, {});
 }
 
+// Re-search missing art --------------------------------------------------
+
+export type ResearchMissingArtResponse = { version: number };
+
+// Bumps cover_art_sources_meta.current_version. Every 'none' row becomes
+// eligible for retry against the current provider chain on the next
+// enrichment pass. Existing positively-sourced rows are not affected.
+export async function researchMissingArt(): Promise<ResearchMissingArtResponse> {
+  return api.post<ResearchMissingArtResponse>('/api/admin/cover-sources/research', {});
+}
+
 // Settings rarely change in operator time. 60s staleTime; refetches
 // on window focus + after any mutation rather than polling.
 export function createCoverProvidersQuery() {