Feature Request: Add WebSocket subscription for whale alerts

## Feature Request

### Description
Add a WebSocket subscription endpoint that allows clients to receive real-time notifications for large STX transfers (whale alerts).

### Use Case
DeFi monitoring applications need to track large transactions in real-time. Currently, this requires polling the API or setting up Chainhooks separately.

### Proposed Solution
Add a new WebSocket channel:
```json
{
"type": "subscribe",
"channel": "whale-alerts",
"threshold": 100000000000 // 100K STX in microSTX
}
```

### Benefits
- Reduces API polling load
- Enables real-time DeFi monitoring
- Improves developer experience

### Reference
I've implemented a similar feature in my project: https://github.com/serayd61/stacks-defi-sentinel

Would be happy to contribute if this feature is accepted.