Fix TypeScript: add @types/ws and RawData type for WebSocket message

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

backend/src/routes/ws.ts

@@ -1,6 +1,6 @@
 import { FastifyPluginAsync } from 'fastify'
 import fastifyWebsocket from '@fastify/websocket'
-import type { WebSocket } from 'ws'
+import type { WebSocket, RawData } from 'ws'
 
 // hotel slug → set of connected clients
 const hotelRooms = new Map<string, Set<WebSocket>>()
@@ -18,7 +18,7 @@ const ws: FastifyPluginAsync = async (fastify) => {
       if (!hotelRooms.has(hotel)) hotelRooms.set(hotel, new Set())
       hotelRooms.get(hotel)!.add(socket)
 
-      socket.on('message', (raw) => {
+      socket.on('message', (raw: RawData) => {
         const data = raw.toString()
         const peers = hotelRooms.get(hotel)
         if (!peers) return