Stream Prediction Confidence

Stream the confidence result for prediction_id over SSE.

Emits a status event on subscribe (queued or completed) and a terminal completed / failed event when the worker finishes. Sends a comment line every STREAM_KEEPALIVE_INTERVAL_S seconds so idle proxies don't drop the connection. Closes after STREAM_WALL_CAP_S seconds with a final status: queued event so the client reconnects instead of holding open forever.

Returns 404 if neither a CH row, a pending Redis prediction, nor a Celery task exists for prediction_id. Returns 403 if the caller lacks read access on the arbiter repo.