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/DataController.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Support\RigData;
+use Illuminate\Http\Request;
+use Illuminate\Http\Response;
+use Symfony\Component\HttpFoundation\BinaryFileResponse;
+use Symfony\Component\HttpFoundation\Response as SymfonyResponse;
+
+class DataController extends Controller
+{
+    /**
+     * Serve the playlist as a script-tag-friendly JS global. Cached via ETag on file mtime
+     * so CEF can revalidate cheaply (the playlist file changes rarely but the body is ~142 KB).
+     */
+    public function playlist(Request $request, RigData $rig)
+    {
+        $path = $rig->playlistPath();
+        $tracks = $rig->playlist();
+        $mtime = is_file($path) ? filemtime($path) : 0;
+        $etag = '"' . md5('playlist:' . $mtime) . '"';
+
+        if ($request->headers->get('If-None-Match') === $etag) {
+            return response('', 304)->header('ETag', $etag);
+        }
+
+        // Rewrite `file://` URIs into `/audio?p=...` HTTP URLs so the
+        // music-daemon page (served over HTTP) can load tracks. Chromium
+        // blocks HTTP-origin pages from loading file:// media.
+        if (isset($tracks['tracks']) && is_array($tracks['tracks'])) {
+            foreach ($tracks['tracks'] as $i => $track) {
+                if (!isset($track['file'])) continue;
+                if (str_starts_with($track['file'], 'file://')) {
+                    $absolute = rawurldecode(substr($track['file'], 7));
+                    $tracks['tracks'][$i]['file'] = \App\Http\Controllers\AudioController::urlFor($absolute);
+                }
+            }
+        }
+
+        $body = 'window.__PLAYLIST = ' . json_encode(
+            $tracks,
+            JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE
+        ) . ';';
+
+        return response($body, 200, [
+            'Content-Type'  => 'application/javascript; charset=utf-8',
+            'Cache-Control' => 'public, must-revalidate',
+            'ETag'          => $etag,
+        ]);
+    }
+
+    /**
+     * Serve bridges/cover.jpg. `no-store` so CEF never holds a stale image
+     * even if a caller forgets the ?v=<hash> cache-bust. 404 returns a 1×1
+     * transparent PNG so the music-box cover placeholder doesn't get a
+     * broken-image icon while the bridge is starting up.
+     */
+    public function coverImage(RigData $rig): SymfonyResponse
+    {
+        $path = $rig->coverPath();
+        if (is_file($path) && is_readable($path)) {
+            return response()->file($path, [
+                'Content-Type'  => 'image/jpeg',
+                'Cache-Control' => 'no-store, max-age=0',
+            ]);
+        }
+
+        // 1x1 transparent PNG (67 bytes).
+        $png = base64_decode(
+            'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYAAAAAYAAjCB0C8AAAAASUVORK5CYII='
+        );
+        return response($png, 200, [
+            'Content-Type'  => 'image/png',
+            'Cache-Control' => 'no-store, max-age=0',
+        ]);
+    }
+}