Withdraw from the contract (consumers)

Records the customer's withdrawal and cancels the subscription immediately. A withdrawal is a statement the customer makes, not a request we grant, so it is recorded even outside the 14 days — within_window says whether it was late. A reason is optional by law and is never required. The refund amount is computed automatically from what Stripe actually charged and returned in refund; the Stripe refund itself is still issued by hand, to the original payment method, within 14 days.