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/index.php

@@ -0,0 +1,28 @@
+<?php
+declare(strict_types=1);
+require_once dirname(__DIR__) . '/src/bootstrap.php';
+
+/* ── Simple front-controller ─────────────────────────────────────── */
+$uri    = parse_url($_SERVER['REQUEST_URI'] ?? '/', PHP_URL_PATH);
+$script = basename($uri);
+
+/* Strip project prefix /app/androidcast_project/sfu */
+$uri = preg_replace('#^/app/androidcast_project/sfu#', '', $uri);
+
+$routes = [
+    '/'           => 'dashboard.php',
+    '/health'     => 'health.php',
+    '/health.php' => 'health.php',
+    '/api/rooms'  => 'api/rooms.php',
+    '/api/join'   => 'api/join.php',
+    '/api/stats'  => 'api/stats.php',
+];
+
+$target = $routes[$uri] ?? null;
+if ($target && is_file(__DIR__ . '/' . $target)) {
+    require __DIR__ . '/' . $target;
+    exit;
+}
+http_response_code(404);
+header('Content-Type: application/json');
+echo json_encode(['ok' => false, 'error' => 'not found']);