Server-Sent Events for Job Progress

Stream job progress to the browser with Server-Sent Events. The job itself keeps running if the user closes the tab. JOB What it does: [IMPORT | EXPORT | REPORT] How progress is stored: [DB ROW] Auth: [COOKIE SESSION] Proxy in front: [NGINX | A PLATFORM THAT BUFFERS — NAME IT] DELIVER 1) The response headers: text/event-stream, no buffering, retry field. The platform-specific setting that disables proxy buffering. 2) The event types: progress (done, total), message, done, error. Each with a JSON data line and an id so the client can resume with Last-Event-ID. 3) The server loop: poll or listen, send, and stop when the job is finished or the request is aborted. Show the abort handler so we do not leak the loop. 4) The client: EventSource, the UI states, and why we do not use it for the request that starts the job (a normal POST starts it, then the stream watches it). 5) Reconnect: the job is the source of truth, so a new stream catches up from the row, not from memory. Do not hold the database transaction open for the life of the stream.