Add refund

payment.py

class Payment:
    # ... existing methods
    
    def refund(self, amount_refunded: float, refund_id: str) -> None:
        # business rules
        if self.status != 'SETTLED':
            raise Exception('Only settled payments can be refunded')

        if amount_refunded > self.amount_due:
            raise Exception('You can only refund amounts equal or smaller than the amount due')
        
        if amount_refunded <= 0:
            raise Exception('You can only refund amounts greater than zero')
        
        self.record_that(PaymentRefunded(
            self.payment_id, amount_refunded, refund_id)
        )

    def apply_payment_refunded(self, event: PaymentRefunded):
        self._status = 'REFUNDED'
        self._amount_due -= event.amount_refunded

    def record_that(self, event: DomainEvent):
        if isinstance(event, PaymentCreated):
            self.apply_payment_created(event)
        elif isinstance(event, PaymentRefunded):
            self.apply_payment_refunded(event)
            
        self.events.append(event)