fix: prevent race condition on order subtotal calculation

internal/processor/order_processor.go

@@ -338,7 +338,7 @@ func (p *OrderProcessorImpl) AddToOrder(ctx context.Context, orderID uuid.UUID,
 			ProductID:        itemReq.ProductID,
 			ProductVariantID: itemReq.ProductVariantID,
 			Quantity:         itemReq.Quantity,
-			UnitPrice:        unitPrice, // Use price from database
+			UnitPrice:        unitPrice,
 			TotalPrice:       itemTotalPrice,
 			UnitCost:         unitCost,
 			TotalCost:        itemTotalCost,
@@ -594,6 +594,10 @@ func (p *OrderProcessorImpl) VoidOrder(ctx context.Context, req *models.VoidOrde
 				return fmt.Errorf("order item does not belong to this order")
 			}
 
+			if orderItem.Status == entities.OrderItemStatusCancelled {
+				return fmt.Errorf("order item %s is already cancelled", orderItemID)
+			}
+
 			if itemVoid.Quantity > orderItem.Quantity {
 				return fmt.Errorf("void quantity cannot exceed original quantity for item %d", itemVoid.OrderItemID)
 			}
@@ -614,9 +618,15 @@ func (p *OrderProcessorImpl) VoidOrder(ctx context.Context, req *models.VoidOrde
 			return fmt.Errorf("outlet not found: %w", err)
 		}
 
+		// Reload order to get latest state
+		order, err = p.orderRepo.GetByID(ctx, req.OrderID)
+		if err != nil {
+			return fmt.Errorf("failed to reload order: %w", err)
+		}
+
 		order.Subtotal -= totalVoidedAmount
 		order.TotalCost -= totalVoidedCost
-		order.TaxAmount = order.Subtotal * outlet.TaxRate // Recalculate tax using outlet's tax rate
+		order.TaxAmount = order.Subtotal * outlet.TaxRate
 		order.TotalAmount = order.Subtotal + order.TaxAmount - order.DiscountAmount
 
 		if err := p.orderRepo.Update(ctx, order); err != nil {