Join a chatroom.

/join is the single source of truth for "becoming joinable": if the room is still scheduled but within its start window (15 min before scheduled_at onward), it is transitioned to active below — AFTER the private-room gate — so every entry point (CommuniTea cards, shared invite links, the calendar, future callers) works whether or not it remembered to call /start first. Previously only some frontend paths called /start, so the rest 404'd with "Chatroom not found or not active" on a perfectly valid scheduled room.