BuyGiftCard and till online_square charge the cnon: nonce directly (no synthetic card-on-file); BuyGiftCard.IdempotencyKey is now validate-required (empty key previously collided on the UNIQUE constraint). Saved-card branches forward customer_id and lazily provision legacy cards. lockCancellationPayments uses the deliberately-blocking xact lock so a cancellation never silently drops a refund under contention (admins see the full manual refund round-trip).