Add Express server with WebSocket, CORS, and room routes

Set up the HTTP server with Express, attach Socket.IO for real-time
messaging, add CORS middleware, async error handling utility, and
REST routes for room join/leave/messages/participants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Backend/middleware/cors.js

@@ -0,0 +1,15 @@
+function createCorsMiddleware(clientOrigin) {
+  return (req, res, next) => {
+    res.setHeader('Access-Control-Allow-Origin', clientOrigin)
+    res.setHeader('Access-Control-Allow-Methods', 'GET,POST,PATCH,DELETE,OPTIONS')
+    res.setHeader('Access-Control-Allow-Headers', 'Content-Type')
+
+    if (req.method === 'OPTIONS') {
+      return res.sendStatus(204)
+    }
+
+    next()
+  }
+}
+
+module.exports = createCorsMiddleware