Turn a finished livestream into a VOD. The server concatenates the MUXL segments it recorded for the livestream into a single content blob, derives the playback sidecars, and publishes the place.stream.media.track records. Processing completes server-side and creates a draft VOD; the client does not need to poll or publish — the user publishes the draft later via place.stream.vod.publishDraft. Returns the draft's ats:// URI so the client can navigate to it, alongside an uploadId for backwards compatibility.