Say that a session is happening

One occurrence, one row: a weekly club ride is entered each week. There is no recurrence rule, on purpose — a stored schedule outlives its own truth, and when the club moves to Wednesdays the app would be confidently wrong for months. Adding one never replaces anything: unlike planning the engine's own session, a second commitment on the same day and sport is a second session, because two of the athlete's arrangements can share a day. Sending the same commitment twice stores one: the second call answers with the first one's id and writes nothing. Bounded, whoever is writing: no more than 180 days ahead or a day back (OUT_OF_HORIZON, 400), at most three on one day (DATE_OCCUPIED, 409), and at most 750 standing on the calendar (SCHEDULE_FULL, 409 — waiting does not help; withdraw one). A connected app is also held to a rolling per-app quota (QUOTA_EXCEEDED, 429, with retryAfterS); the athlete's own session is not.