Channel-based WebSocket transport for togo realtime (implements `togo.Broker`,
overrides the default SSE broker when installed).

```bash
togo install togo-framework/realtime-ws
```

## Channels & broadcasting
- Clients subscribe over the socket: `{"action":"subscribe","channel":"orders"}`.
- **Public** channels: any name. **Private** channels: prefix `private-` (require a ticket).
- Broadcast to a channel by emitting `"channel:event"` (e.g. `app.Emit(ctx, "orders:created", o)`);
  a plain event (no `:`) goes to everyone.

## Tickets (private channels)
`GET /events/ticket` returns a short-lived HMAC-signed ticket — **mount it behind your
auth** so only authenticated users get one. The client connects with `/events?ticket=...`;
a valid ticket unlocks `private-*` subscriptions. Set `REALTIME_SECRET`.

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