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/scenes.blade.php

@@ -85,7 +85,6 @@
 
   <div class="group-title">DATA ENDPOINTS</div>
   <ul>
-    <li><a class="route" href="/data/playlist.js">/data/playlist.js</a> <span class="note">— window.__PLAYLIST</span></li>
     <li><a class="route" href="/cover.jpg">/cover.jpg</a> <span class="note">— current album art</span></li>
     <li><a class="route" href="/track">/track?p=&lt;base64&gt;</a> <span class="note">— audio stream</span></li>
   </ul>