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/app.ts

@@ -10,6 +10,7 @@ import { join } from 'path'
 import { config } from './config'
 import './types' // side-effect: augments fastify types
 
+import fastifyWebsocket   from '@fastify/websocket'
 import wsRoutes          from './routes/ws'
 import authRoutes        from './routes/auth'
 import hotelsRoutes      from './routes/hotels'
@@ -53,6 +54,9 @@ export async function buildApp() {
   const uploadsDir = process.env.UPLOADS_DIR ?? join(process.cwd(), 'uploads')
   await fastify.register(staticFiles, { root: uploadsDir, prefix: '/uploads/' })
 
+  // ── WebSocket ──────────────────────────────────────────────────────────────
+  await fastify.register(fastifyWebsocket)
+
   // ── Security ───────────────────────────────────────────────────────────────
   await fastify.register(helmet, { contentSecurityPolicy: false, crossOriginResourcePolicy: { policy: 'cross-origin' } })
 
@@ -114,7 +118,7 @@ export async function buildApp() {
   await fastify.register(loyaltyRoutes)
   await fastify.register(chatRoutes)
   await fastify.register(workstationRoutes)
-  setupAgentWsRoute(fastify)
+  await fastify.register(setupAgentWsRoute)
 
   startJobs()