feat(web): tag enrichment sources admin card (#1490 Step 4 web)

Add a "Tag enrichment sources" card to the admin integrations page,
mirroring the cover-art providers card: per-provider enable toggle +
API-key field + Save + Test connection, over /api/admin/tag-sources.
Enabling/keying a source (e.g. pasting a Last.fm key) re-opens settled
tracks for re-enrichment via the version bump.

- admin.ts: TagProvider types + get/update/test functions +
  createTagProvidersQuery; qk.tagProviders key.
- integrations/+page.svelte: the card + local edit state, with
  MusicBrainz (keyless baseline) and Last.fm (needs a free key) notes.
- Tests: admin.tag-sources API test + integrations component tests
  (render / enable+key save / test connection), plus the mock plumbing
  the existing suite needs for the new query.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/api/queries.ts

@@ -48,6 +48,7 @@ export const qk = {
   scanStatus:             ()              => ['scanStatus'] as const,
   coverage:               ()              => ['coverage'] as const,
   coverProviders:         ()              => ['coverProviders'] as const,
+  tagProviders:           ()              => ['tagProviders'] as const,
   adminUsers:             ()              => ['adminUsers'] as const,
   adminInvites:           ()              => ['adminInvites'] as const,
   adminDiagnostics:       (f: Record<string, string | number | undefined>) =>