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/api/stats.php

@@ -0,0 +1,25 @@
+<?php
+declare(strict_types=1);
+/**
+ * SFU stats ingest — POST /api/stats
+ * Body: { participant_id, bitrate_kbps, packet_loss }
+ * Called periodically by Android client (every 5s) to feed analytics.
+ */
+require_once dirname(__DIR__, 2) . '/src/bootstrap.php';
+
+if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+    sfuJsonErr('method not allowed', 405);
+}
+
+sfuRequireAuth();
+$input = json_decode(file_get_contents('php://input'), true) ?? [];
+
+$pid  = (int)   ($input['participant_id'] ?? 0);
+$bw   = (float) ($input['bitrate_kbps']  ?? 0.0);
+$pl   = (float) ($input['packet_loss']   ?? 0.0);
+
+if ($pid <= 0) { sfuJsonErr('participant_id required'); }
+
+$repo = new \AndroidCast\Sfu\SfuRoomRepository(sfuPdo());
+$repo->recordStats($pid, $bw, $pl);
+sfuJsonOk(['recorded' => true]);