Subscribe to live call events (SSE)

Real-time streaming of user speech (STT) and agent speech (TTS) events for an active call via Server-Sent Events. The connection is real-time — events stream directly from the call runtime as they are produced. The SSE connection auto-closes when the call ends (`sse_close` event). Only active calls can be subscribed to; completed calls return a 400 error. **Transcript event types:** - `user_interim_transcription` — Partial, in-progress transcription as the user speaks. Use for live preview only; will be superseded by `user_transcription`. - `user_transcription` — Final transcription for a completed user speech turn. - `tts_completed` — Fired when the agent finishes speaking a TTS segment. Includes the spoken text and optionally TTS latency. **Lifecycle events:** - `sse_init` — Sent immediately when the SSE connection is established. - `sse_close` — Sent when the call ends, right before the server closes the connection. Other event types (e.g. `call_start`, `call_end`, `turn_latency`, metrics) are also sent on this stream.