feat(api): admin tag-sources endpoints (#1490 Step 4 backend)

Expose the tag-enrichment provider settings over the admin API, mirroring
the cover-sources surface so a Last.fm key can be pasted and sources
toggled from the web admin UI (rules #25/#27).

- GET  /api/admin/tag-sources                 — list providers + version
- PATCH/api/admin/tag-sources/{id}            — enable / set api_key
- POST /api/admin/tag-sources/{id}/test       — test connection
- POST /api/admin/tag-sources/research        — bump version, re-open
                                                 settled rows for re-enrich
- Thread tags.SettingsService through server.New (struct field, like
  RecSettings) → Router → api.Mount → handlers.tagSettings; main.go sets
  srv.TagSettings.

Handler tests mirror admin_cover_sources_test (list / flip-bumps-version /
key-only-no-bump / unknown-404 / non-admin-403 / not-testable-ok-false),
integration-tier (skip without MINSTREL_TEST_DATABASE_URL).

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

cmd/minstrel/main.go

@@ -326,6 +326,7 @@ func run() error {
 	srv.Bus = bus
 	srv.PlaylistScheduler = playlistScheduler
 	srv.RecSettings = recSettings
+	srv.TagSettings = tagSettings
 	srv.StreamSecret = cfg.StreamSecret
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,