feat(server/m7-353): admin endpoints for cover refetch (per-album + bulk)

Adds POST /api/admin/albums/{id}/cover/refetch (synchronous single-album
retry) and POST /api/admin/covers/refetch-missing (async bulk drain).
Wires coverart.Enricher through server.New → api.Mount → handlers struct.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -134,7 +134,7 @@ func run() error {
 
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding)
+	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap)
 	playlists.StartSystemPlaylistCron(ctx, pool, logger.With("component", "system_playlist_cron"))
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,