SSE stream of job progress and final results

Server-Sent Events stream for real-time job tracking. Preferred over polling for Web UI.

Event types:

SSE (not WebSocket) because this is a unidirectional server→client stream. SSE auto-reconnects, passes through proxies, and works with the native EventSource browser API.

const es = new EventSource("/api/v1/query/jobs/qry_xxx/stream");
es.addEventListener("progress", (e) => updateProgressBar(JSON.parse(e.data)));
es.addEventListener("complete", (e) => { showFinalResults(JSON.parse(e.data)); es.close(); });
es.addEventListener("failed",   (e) => { showError(JSON.parse(e.data)); es.close(); });