webapp: introduce Laravel app under webapp/ for scene overlays

Replaces the per-scene HTML directories (landing/, loading/, game/,
desktop/, goodbye/, music-box/, music/) with a single Laravel app
serving every overlay over HTTP. Supervisord runs `php artisan serve`
on 127.0.0.1:1118 and the OBS scene JSON now references HTTP routes
instead of file:// URLs.

Highlights:
 - public/css/hud.css consolidates the duplicated HUD chrome,
   scanlines/vignette/flicker, terminal styling, and pulse keyframes
   that were copy-pasted across all seven scenes.
 - Blade partials own hud-strip, crt-overlays, obs-ws-scripts,
   camera-frame, screen-frame; the seven scenes extend a shared
   overlay layout.
 - Artisan commands (`rig:setup`, `rig:telemetry`, `rig:loading`,
   `rig:playlist`, `rig:cmd`) replace the shell scripts that wrote
   per-rig JSON snapshots. TwitchHelix + HardwareSnapshot services
   handle the work the bash + Python helpers used to.
 - ObsWsClient + MusicCommandController kill the 250 ms cmd.js poll
   in the music daemon: POST /cmd/{skip|prev|pause|resume} opens a
   short-lived Pawl WS, authenticates, and broadcasts mpd:cmd.
 - AudioController streams files from the configured music dirs so
   CEF can load tracks under the HTTP origin (Chromium blocks
   HTTP-origin pages from loading file:// media).
 - DataController serves /data/playlist.js (with ETag mtime cache)
   and /cover.jpg (no-store) so the existing overlays' window.__PLAYLIST
   and cover.jpg cache-bust pattern keeps working.

scripts/obs-webapp.supervisord.conf is the supervisord unit; install
to /etc/supervisor.d/obs-webapp.conf.
scripts/rewrite-scene-urls.py is a one-shot tool that rewrites
basic/scenes/Default_Stream_HUD.json from file:// to HTTP URLs.
basic/scenes/Default_Stream_HUD.json.pre-webapp is the rollback
artifact (made with OBS closed; full pre-migration state).

The seven old scene directories, vendor/, and the bash scripts are
still on disk pending visual verification; the next commit will
prune them.

webapp/app/Http/Controllers/AudioController.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\BinaryFileResponse;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * Streams audio files from the configured music_dirs via HTTP so the
+ * music-daemon page (served on http://127.0.0.1:8000/) can load them.
+ *
+ * Chromium blocks HTTP-origin pages from loading `file://` media as a
+ * mixed-protocol security policy, which is why the old file://-only
+ * setup worked but the new HTTP setup needs this.
+ *
+ * Path-traversal guard: the resolved file's real path must sit inside
+ * one of the configured roots; the request 404s otherwise.
+ */
+class AudioController extends Controller
+{
+    public function stream(Request $request): Response
+    {
+        $b64 = (string) $request->query('p', '');
+        if ($b64 === '') abort(400, 'missing p');
+
+        // URL-safe base64; pad back to a multiple of 4 for strict decoding.
+        $padded = $b64 . str_repeat('=', (4 - strlen($b64) % 4) % 4);
+        $path = base64_decode(strtr($padded, '-_', '+/'), true);
+        if ($path === false || $path === '') abort(400, 'bad encoding');
+
+        $real = realpath($path);
+        if ($real === false || !is_file($real)) abort(404, 'file not found: ' . $path);
+
+        $allowed = false;
+        foreach ((array) config('rig.music_dirs') as $root) {
+            $rootReal = realpath($root);
+            if ($rootReal && str_starts_with($real . '/', rtrim($rootReal, '/') . '/')) {
+                $allowed = true;
+                break;
+            }
+        }
+        if (!$allowed) abort(403, 'outside allowed roots');
+
+        $ext = strtolower(pathinfo($real, PATHINFO_EXTENSION));
+        $mime = match ($ext) {
+            'm4a', 'aac' => 'audio/mp4',
+            'mp3'        => 'audio/mpeg',
+            'opus', 'ogg'=> 'audio/ogg',
+            'flac'       => 'audio/flac',
+            'webm'       => 'audio/webm',
+            'wav'        => 'audio/wav',
+            default      => 'application/octet-stream',
+        };
+
+        // BinaryFileResponse handles Range requests automatically, which the
+        // HTML5 <audio> element issues when seeking.
+        $resp = new BinaryFileResponse($real, 200, [
+            'Content-Type'  => $mime,
+            'Accept-Ranges' => 'bytes',
+            'Cache-Control' => 'public, max-age=86400',
+        ]);
+        return $resp;
+    }
+
+    /** Encode an absolute path into the URL-safe base64 used by the {@see stream} route. */
+    public static function urlFor(string $absolutePath): string
+    {
+        return '/track?p=' . rtrim(strtr(base64_encode($absolutePath), '+/', '-_'), '=');
+    }
+}