Implement Real-Time Features with WebSockets

Implement real-time functionality using WebSockets for the following feature:

Feature: [e.g. live chat / real-time notifications / collaborative editing / live dashboard / presence indicators]
Backend: [Node.js with ws/Socket.io / Python with FastAPI WebSockets / Go]
Frontend: [React / Vanilla JS / Next.js]
Scale expectation: [single server / needs horizontal scaling]

Implement:
1. Server-side WebSocket handler:
   - Connection management (store active connections)
   - Message routing / event types
   - Broadcasting (to all / to room / to specific user)
   - Disconnection cleanup
   - Heartbeat / ping-pong to detect dead connections

2. Client-side connection:
   - Connection with auto-reconnect and exponential backoff
   - Event listeners for each message type
   - Connection state management (connecting / connected / disconnected)
   - Graceful cleanup on component unmount

3. Authentication: how to authenticate a WebSocket connection

4. Message format: standardised event envelope

5. If scaling beyond one server: how to use Redis pub/sub for cross-instance broadcasting

Include error handling for network interruptions.