A report generation's status, progress and outcome

Progress ({step, sectionsDone, sectionsTotal, message}) is written at most every 2 s while the generation is pending or running; poll every 2 s. Once finished it carries its result (draft_ready) or a user-worded error (failed). A generation that stopped responding is ended failed (STALE) when read. 404 when REPORTS_V2_ENABLED is off, and for a generation of another organization or workspace. Needs report.create.