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/music/box.blade.php

@@ -27,7 +27,6 @@
 
 @push('data-scripts')
 <script>window.__TRACK_COUNT = @json($trackCount);</script>
-<script src="{{ asset('data/playlist.js') }}"></script>
 @endpush
 
 @section('content')
@@ -72,8 +71,7 @@
   'use strict';
   const pad = HUD.pad;
 
-  const playlistData = window.__PLAYLIST || { tracks: [] };
-  const trackCount = (playlistData.tracks || []).length || (window.__TRACK_COUNT || 0);
+  const trackCount = window.__TRACK_COUNT || 0;
   document.getElementById('rig').textContent =
     trackCount > 0 ? `LIBRARY :: ${trackCount} TRACKS` : '— EMPTY LIBRARY —';