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/admin.ts

@@ -383,6 +383,67 @@ export function createCoverProvidersQuery() {
   });
 }
 
+// Tag-enrichment providers -------------------------------------------------
+// Parallel to the cover-art providers surface, over /api/admin/tag-sources.
+// Independent settings so a new folksonomy source is added without touching
+// art config (#1490).
+
+export type TagProviderCapability = 'track_tags';
+
+export type TagProvider = {
+  id: string;
+  display_name: string;
+  requires_api_key: boolean;
+  supports: TagProviderCapability[];
+  enabled: boolean;
+  api_key_set: boolean;
+  display_order: number;
+  testable: boolean;
+};
+
+export type TagProvidersResponse = {
+  providers: TagProvider[];
+  sources_version: number;
+};
+
+export async function getTagProviders(): Promise<TagProvidersResponse> {
+  return api.get<TagProvidersResponse>('/api/admin/tag-sources');
+}
+
+export type UpdateTagProviderPatch = {
+  enabled?: boolean;
+  api_key?: string;
+};
+
+export type UpdateTagProviderResponse = TagProvider & {
+  version_bumped: boolean;
+};
+
+export async function updateTagProvider(
+  id: string,
+  patch: UpdateTagProviderPatch
+): Promise<UpdateTagProviderResponse> {
+  return api.patch<UpdateTagProviderResponse>(`/api/admin/tag-sources/${id}`, patch);
+}
+
+export type TestTagProviderResponse = {
+  ok: boolean;
+  duration_ms?: number;
+  error?: string;
+};
+
+export async function testTagProvider(id: string): Promise<TestTagProviderResponse> {
+  return api.post<TestTagProviderResponse>(`/api/admin/tag-sources/${id}/test`, {});
+}
+
+export function createTagProvidersQuery() {
+  return createQuery({
+    queryKey: qk.tagProviders(),
+    queryFn: getTagProviders,
+    staleTime: 60_000
+  });
+}
+
 // Admin user-management ------------------------------------------------------
 
 export type AdminUser = {