Reapply "feat(loyalty): EnakPoint & EnakCoin" (#32)

This reverts commit 4e24f9bbb0.

internal/service/order_service.go

@@ -557,7 +557,8 @@ func (s *OrderServiceImpl) validateCreatePaymentRequest(req *models.CreatePaymen
 		return fmt.Errorf("payment method ID is required")
 	}
 
-	if req.Amount <= 0 {
+	// A payment with EnakPoint gives points instead; its amount is computed from them.
+	if req.Points == nil && req.Amount <= 0 {
 		return fmt.Errorf("payment amount must be greater than zero")
 	}