Take everything one app scheduled off the calendar

The remedy for an assistant that misunderstood: every standing session the app wrote is withdrawn, the same soft withdrawal as POST /commitments/{id}/withdraw, and every event it wrote is removed, as DELETE /events/{id} would, except one the athlete has since made an A event — that letter builds a season around the date, and choosing it is the athlete taking the event as their own. Nothing the athlete entered themselves is touched, and nothing another app wrote. Works after the app is disconnected: disconnecting stops future writes and leaves the calendar alone, and this is the separate act that clears it. Answers with counts, and with zeros rather than an error when there was nothing to do, so a repeat is harmless. Takes the client id in the body rather than the path because a client id can be a URL.