SSE endpoint streaming each stdout/stderr line from a detached job as it's produced. Mirrors the `Command.logs()` async iterator shape on `@vercel/sandbox` — events carry `{ stream, data }`.

Late subscribers only see events produced after they connect. The JobState snapshot (GET /jobs/{job_id}) covers the history.

A "done" sentinel event fires when the broadcast channel closes (the exec task has exited and dropped the sender), signalling callers they can stop reading.