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/utils/asyncHandler.js

@@ -0,0 +1,7 @@
+function asyncHandler(handler) {
+  return (req, res, next) => {
+    Promise.resolve(handler(req, res, next)).catch(next)
+  }
+}
+
+module.exports = asyncHandler