sfu-signaling: real Janus signaling service implementation

- JanusClient.php: HTTP REST client for Janus Gateway API
- SfuRoomRepository.php: room + participant + stats persistence
- bootstrap.php: PDO, auth, Janus client factory
- public/index.php: front-controller routing
- public/health.php: Janus online check
- public/api/rooms.php: list/create/destroy rooms
- public/api/join.php: SDP offer relay → Janus VideoRoom
- public/api/stats.php: bitrate/packet-loss analytics ingest
- public/dashboard.php: SFU rooms UI (common BE dark-theme styles)

Co-authored-by: Cursor <cursoragent@cursor.com>

public/health.php

@@ -0,0 +1,22 @@
+<?php
+declare(strict_types=1);
+require_once dirname(__DIR__) . '/src/bootstrap.php';
+
+header('Content-Type: application/json; charset=utf-8');
+
+$janus  = janusClient();
+$info   = $janus->info();
+$online = isset($info['janus']) && $info['janus'] === 'server_info';
+
+echo json_encode([
+    'ok'    => true,
+    'sfu'   => [
+        'enabled'        => true,
+        'status'         => $online ? 'online' : 'unreachable',
+        'janus_version'  => $info['version_string'] ?? null,
+        'janus_url'      => sfuConfig('janus_url', 'http://10.7.16.237:8088/janus'),
+        'signaling_url'  => sfuConfig('signaling_url', 'wss://apps.f0xx.org/app/androidcast_project/sfu/ws'),
+        'ws_url'         => '/app/androidcast_project/sfu/ws',
+        'api_base'       => '/app/androidcast_project/sfu/api',
+    ],
+], JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);