preview: a share link that lets somebody in - an admin's group page can make a letting-in link beside the ordinary Share link, and whoever opens it taps once and is in the chat with nobody asked. One person, seven days, revocable, and a spent or revoked link falls back to an ordinary knock an admin answers. ONE-WAY DOOR: schema 21 to 22, so an account opened by this build or anything newer cannot be opened by v0.1.30-pre.60 or older.