QA recordings

Every automated browser test records what the page did as an rrweb DOM recording, not a video. The chunks are ordinary private files in the qa-recordings store, so the files commands work on them directly.

ls lists recording sessions grouped into runs. export renders a window of one session to an mp4 for a changelog, a release note, or a bug report. Export renders on the dev box with a real browser and ffmpeg. It works only inside a sandbox, blocks for the length of the clip, and won't start while another export is running.