Job progress as Server-Sent Events

Server-Sent Events endpoint for real-time log streaming.

Emits a comment-line keepalive every _SSE_KEEPALIVE_SEC seconds so reverse proxies (Cloudflare Tunnel, nginx) don't tear down the connection during long quiet stretches of a multi-minute job. Module-level so tests can shrink the interval to a fraction of a second (hostile peer-review R18).

R2-6 — reconnects no longer replay the whole log: every log event carries an SSE id: (its index), browser auto-reconnects send it back as Last-Event-ID, and manual reconnects (e.g. after a token refresh, where the browser does not send the header) can pass ?from=<count already received>.