feat(realtime): Socket.IO Redis adapter — enable horizontal scaling

- RedisIoAdapter wires @socket.io/redis-adapter with dedicated pub/sub clients
- channel key 'tripz:sio' isolates on shared Redis
- wired in main.ts via useWebSocketAdapter; multiple app replicas can now share WS connections

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/package.json

@@ -29,6 +29,7 @@
     "@nestjs/throttler": "^6.2.0",
     "@nestjs/typeorm": "^11.0.0",
     "@nestjs/websockets": "^11.0.0",
+    "@socket.io/redis-adapter": "^8.3.0",
     "bullmq": "^5.12.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.1",