fix: register @fastify/websocket globally so /ws/agent route works

setupAgentWsRoute was called outside the encapsulated wsRoutes plugin
scope where @fastify/websocket was registered. Register the plugin at
the root level and convert setupAgentWsRoute to FastifyPluginAsync.

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

backend/src/routes/ws.ts

@@ -1,5 +1,4 @@
 import { FastifyPluginAsync } from 'fastify'
-import fastifyWebsocket from '@fastify/websocket'
 import type { SocketStream } from '@fastify/websocket'
 import type { RawData } from 'ws'
 
@@ -18,8 +17,6 @@ export function broadcast(hotelSlug: string, message: object) {
 const PING_INTERVAL_MS = 25_000 // ping every 25s — keeps nginx proxy_read_timeout alive
 
 const ws: FastifyPluginAsync = async (fastify) => {
-  await fastify.register(fastifyWebsocket)
-
   fastify.get<{ Querystring: { hotel?: string } }>(
     '/ws',
     { websocket: true },