VoiceBlender Streaming Interface (VSI)

Upgrades to a WebSocket connection and streams all events in real-time as JSON text frames. The JSON shape is identical to webhook payloads. The server sends a {"type":"connected"} message on connect, followed by events and periodic {"type":"ping"} keepalives. Clients may send {"type":"pong"} or {"type":"stop"} to close gracefully. Unknown message types receive an error response with the echoed request_id (reserved for future commands).