Live generation event stream (NDJSON keep-alive)

Long-lived NDJSON keep-alive stream of generation events. Replays the buffered events from lastId (default: from the start), then tails new ones live. Each line is a JSON object { id, type, data } where type ∈ chunk | flush | media | tldr | toc | metadata | phase | done | error | end. NOT a JSON response — consume as a stream and reconnect with ?lastId= (the last seen id) if the socket drops. Ends on a terminal (done/error) event, when generation is no longer active, on a safety timeout, or on client disconnect.