Stream events (SSE)

Requires scope: read. Server-Sent Events live tail. Each event is a frame id: <event_id>, event: <type>, data: <json>. The stream also says where it is, in an id: line with no data (id: evt_…~<number>), when the connection opens and after each batch of events. That is its position, and what to resume from: a browser's EventSource sends it back as Last-Event-ID by itself, and any other client passes the last one it read as ?since=. Without since or a Last-Event-ID header the stream starts at the newest event: it sends what happens from then on, never the workspace's history; since=0 replays the whole history first. The connection self-closes after ~50s; reconnect to continue. Events arrive in id order, except one whose row was written late: it is sent after newer ones rather than skipped, and a reconnect from the position still gets it. Delivery is at least once: an event can arrive a second time after a reconnect, so de-duplicate by its id. Resuming from a bare event id (since=evt_…) starts after that id: it does not get an event written late with a smaller id, and after such an event it sends the newer ones again.