Stream Call Transcript
Stream a call's transcript via Server-Sent Events.
On connect the server replays all existing transcript turns from the
database, then streams new turns in real time as they arrive from the
voice provider. A ``: heartbeat`` comment is sent every 15 s to keep
proxies alive. The stream closes after sending an ``ended`` event
when the call completes (or immediately after replay if the call has
already ended).
Works for both live and completed calls — same URL either way.

