Stream real-time call transcript via SSE

Server-Sent Events stream of call transcript turns. Each frame is a JSON object with role, text, is_final, turn_id, and ts fields. Normal transcript frames reach the onmessage handler; terminal error frames use event: error. The stream closes when the call ends or the client disconnects. Authenticate via ?token= query param or Authorization: Bearer header (the ?token= form exists because the browser EventSource constructor does not support custom headers).