webapp: drop rig:playlist pipeline, source track count from mpc stats

The four overlays that read /data/playlist.js (loading, goodbye,
music-box, music/nc) only ever consumed `tracks.length` for cosmetic
"LIBRARY :: N TRACKS" / "indexed N tracks" flavor text — all live
playback signal flows through bridges/mpd-state.py over OBS WS. The
142 KB JSON, ffprobe walk, and ETag-cached route were all producing
one integer that MPD itself already knows.

RigData::playlistCount() now shells `mpc stats` and parses "Songs: N",
returning 0 on any failure (treated by callers as empty library).
Removes RigPlaylistCommand, DataController::playlist(), the
/data/playlist.js route, the per-overlay <script src> + window.__PLAYLIST
shims, and the stale storage/data/playlist.json artifact.

AudioController still consumes config('rig.music_dirs') for the HTTP
audio stream — that's orthogonal and stays.

webapp/resources/views/overlays/loading.blade.php

@@ -30,7 +30,6 @@
   window.__TEL     = @json($telemetry);
   window.__TRACK_COUNT = @json($trackCount);
 </script>
-<script src="{{ asset('data/playlist.js') }}"></script>
 @endpush
 
 @section('content')
@@ -142,8 +141,7 @@
     return parts.join(' ');
   }
 
-  const playlistData = window.__PLAYLIST || { tracks: [] };
-  const allTracks    = playlistData.tracks || [];
+  const trackCount = window.__TRACK_COUNT || 0;
   let bootStarted = false;
   let bootFinished = false;
   let lastPlaybackModes = null;
@@ -214,13 +212,13 @@
     lastPlaybackModes = normalizePlayback(s.playback);
 
     await typeCommand(formatMusicCommand(s.playback));
-    if (allTracks.length === 0) {
-      append('> ', '[audio] no tracks queued — run `php artisan rig:playlist`', 'term-dim');
+    if (trackCount === 0) {
+      append('> ', '[audio] mpd library empty — check music_directory + run `mpc update`', 'term-dim');
       bootFinished = true;
       return;
     }
 
-    append('> ', `[audio] indexed ${allTracks.length} tracks`, 'term-out');
+    append('> ', `[audio] indexed ${trackCount} tracks`, 'term-out');
     await sleep(180);
     append('> ', `[audio] uplink to daemon @ ophi-118://${rigName.toLowerCase()}.audio.bus`, 'term-out');
     await sleep(180);