Create a reservation

Books an arrival window in a zone for one retailer order. A stall is assigned by soft hold: the active stall holding the fewest reservations that overlap your window and are not yet completed or canceled. Capacity never rejects a booking, so a busy zone stacks reservations on its least loaded stall. Arrival windows must start in the future, start no more than 24 hours ahead, and be between 5 and 120 minutes wide; those three limits are deployment defaults and can be tuned per environment. In this version a reservation is confirmed synchronously, so the response already carries status "confirmed". That confirm is a real status change, so a successful create also fires a reservation.status_changed webhook carrying status "confirmed" for the reservation you just got a 201 for. Expect it; it is not a duplicate. Send the optional Idempotency-Key header to make retries safe: a retry with a key you already used returns the stored reservation with 200 instead of creating a second one. A retry whose zone, retailer, order number, end user name, end user phone, or open link differs from the stored reservation is rejected with 422 IDEMPOTENCY_KEY_REUSED; the arrival window is excluded from that comparison because it can be changed later with PATCH.