feat(order): support weight-based products

Products like fish are sold per weighing (4.2 ons, 5.6 ons), which the
order line could not represent: quantity is INTEGER and prices are always
computed as quantity * unit_price.

Model one weighing as one order line. quantity stays INTEGER and keeps
meaning "how many items"; the measured amount goes into a new nullable
order_items.weight, and the line is priced weight * unit_price. Two
weighings of the same product are two lines, never merged into one.

Keeping quantity integral avoids float comparisons in void, refund and
split bill, where accumulated rounding error would silently misbehave —
"1.4 + 1.4 + 1.4" is not 4.2 in float64, which would leave a fully paid
split-bill item marked unpaid.

BillableQuantity() is now the single place that decides between weight
and count; every price and cost calculation goes through it. Missing one
would bill a 4.2 ons fish as a single ons — wrong money, no error.

Two database constraints back the design: a weighed line always carries a
positive weight, and its quantity is pinned to 1. The latter also makes
void all-or-nothing for weighed lines, so the row-splitting branch can
never produce a zero-weight remainder row.

Also wires product.unit_id through the API, which was previously not
settable at all, and corrects the misleading comment on the request's
unit_price field — that value has never been used; price always comes
from the database.

Design notes and the audit of every price multiplication site are in
docs/rfc-weight-based-products.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/processor/order_processor.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 
+	"apskel-pos-be/internal/constants"
 	"apskel-pos-be/internal/entities"
 	"apskel-pos-be/internal/mappers"
 	"apskel-pos-be/internal/models"
@@ -149,6 +150,33 @@ func NewOrderProcessorImpl(
 	}
 }
 
+// resolveLineQuantity reconciles the requested quantity and weight against how the
+// product is sold, returning the values to store on the order line.
+//
+// For weight-based products a line is exactly one weighing, so quantity is pinned to 1
+// and the weighed amount is what gets priced. This mirrors
+// chk_order_items_weight_single_line in the database.
+func resolveLineQuantity(product *entities.Product, quantity int, weight *float64) (int, *float64, error) {
+	if product.SellBy == constants.SellByWeight {
+		if weight == nil {
+			return 0, nil, fmt.Errorf("product %s is sold by weight and requires a weight", product.Name)
+		}
+		rounded := entities.RoundWeight(*weight)
+		if rounded <= 0 {
+			return 0, nil, fmt.Errorf("weight for product %s must be greater than 0", product.Name)
+		}
+		return 1, &rounded, nil
+	}
+
+	if weight != nil {
+		return 0, nil, fmt.Errorf("product %s is not sold by weight and must not carry a weight", product.Name)
+	}
+	if quantity < 1 {
+		return 0, nil, fmt.Errorf("quantity for product %s must be at least 1", product.Name)
+	}
+	return quantity, nil, nil
+}
+
 func (p *OrderProcessorImpl) CreateOrder(ctx context.Context, req *models.CreateOrderRequest, organizationID uuid.UUID) (*models.OrderResponse, error) {
 	orderNumber, err := p.orderRepo.GetNextOrderNumber(ctx, organizationID, req.OutletID)
 	if err != nil {
@@ -194,25 +222,29 @@ func (p *OrderProcessorImpl) CreateOrder(ctx context.Context, req *models.Create
 			}
 		}
 
-		itemTotalPrice := float64(itemReq.Quantity) * unitPrice
-		itemTotalCost := float64(itemReq.Quantity) * unitCost
-
-		subtotal += itemTotalPrice
-		totalCost += itemTotalCost
+		quantity, weight, err := resolveLineQuantity(product, itemReq.Quantity, itemReq.Weight)
+		if err != nil {
+			return nil, err
+		}
 
 		orderItem := &entities.OrderItem{
 			ProductID:        itemReq.ProductID,
 			ProductVariantID: itemReq.ProductVariantID,
-			Quantity:         itemReq.Quantity,
+			Quantity:         quantity,
+			Weight:           weight,
+			UnitID:           product.UnitID,
 			UnitPrice:        unitPrice,
-			TotalPrice:       itemTotalPrice,
 			UnitCost:         unitCost,
-			TotalCost:        itemTotalCost,
 			Modifiers:        entities.Modifiers(itemReq.Modifiers),
 			Notes:            itemReq.Notes,
 			Metadata:         entities.Metadata(itemReq.Metadata),
 			Status:           entities.OrderItemStatusPending,
 		}
+		orderItem.CalculateTotalPrice()
+		orderItem.CalculateTotalCost()
+
+		subtotal += orderItem.TotalPrice
+		totalCost += orderItem.TotalCost
 
 		orderItems = append(orderItems, orderItem)
 	}
@@ -327,26 +359,30 @@ func (p *OrderProcessorImpl) AddToOrder(ctx context.Context, orderID uuid.UUID,
 			}
 		}
 
-		itemTotalPrice := float64(itemReq.Quantity) * unitPrice
-		itemTotalCost := float64(itemReq.Quantity) * unitCost
-
-		newSubtotal += itemTotalPrice
-		newTotalCost += itemTotalCost
+		quantity, weight, err := resolveLineQuantity(product, itemReq.Quantity, itemReq.Weight)
+		if err != nil {
+			return nil, err
+		}
 
 		orderItem := &entities.OrderItem{
 			OrderID:          orderID,
 			ProductID:        itemReq.ProductID,
 			ProductVariantID: itemReq.ProductVariantID,
-			Quantity:         itemReq.Quantity,
+			Quantity:         quantity,
+			Weight:           weight,
+			UnitID:           product.UnitID,
 			UnitPrice:        unitPrice,
-			TotalPrice:       itemTotalPrice,
 			UnitCost:         unitCost,
-			TotalCost:        itemTotalCost,
 			Modifiers:        entities.Modifiers(itemReq.Modifiers),
 			Notes:            itemReq.Notes,
 			Metadata:         entities.Metadata(itemReq.Metadata),
 			Status:           entities.OrderItemStatusPending,
 		}
+		orderItem.CalculateTotalPrice()
+		orderItem.CalculateTotalCost()
+
+		newSubtotal += orderItem.TotalPrice
+		newTotalCost += orderItem.TotalCost
 
 		addedOrderItems = append(addedOrderItems, orderItem)
 	}
@@ -602,8 +638,9 @@ func (p *OrderProcessorImpl) VoidOrder(ctx context.Context, req *models.VoidOrde
 				return fmt.Errorf("void quantity cannot exceed original quantity for item %d", itemVoid.OrderItemID)
 			}
 
-			voidedAmount := float64(itemVoid.Quantity) * orderItem.UnitPrice
-			voidedCost := float64(itemVoid.Quantity) * orderItem.UnitCost
+			billable := orderItem.BillableQuantityFor(itemVoid.Quantity)
+			voidedAmount := entities.RoundMoney(billable * orderItem.UnitPrice)
+			voidedCost := entities.RoundMoney(billable * orderItem.UnitCost)
 
 			totalVoidedAmount += voidedAmount
 			totalVoidedCost += voidedCost
@@ -720,7 +757,7 @@ func (p *OrderProcessorImpl) RefundOrder(ctx context.Context, id uuid.UUID, req
 				return fmt.Errorf("refund quantity cannot exceed original quantity")
 			}
 
-			refundAmount := float64(refundQuantity) * orderItem.UnitPrice
+			refundAmount := entities.RoundMoney(orderItem.BillableQuantityFor(refundQuantity) * orderItem.UnitPrice)
 			if itemRefund.RefundAmount != nil {
 				refundAmount = *itemRefund.RefundAmount
 			}
@@ -1016,7 +1053,7 @@ func (p *OrderProcessorImpl) createInventoryMovement(ctx context.Context, item *
 		PreviousQuantity: float64(updatedInventory.Quantity + item.Quantity), // Add back the quantity that was subtracted
 		NewQuantity:      float64(updatedInventory.Quantity),
 		UnitCost:         item.UnitCost,
-		TotalCost:        float64(item.Quantity) * item.UnitCost,
+		TotalCost:        entities.RoundMoney(item.BillableQuantity() * item.UnitCost),
 		ReferenceType: func() *entities.InventoryMovementReferenceType {
 			t := entities.InventoryMovementReferenceTypePayment
 			return &t
@@ -1314,7 +1351,7 @@ func (p *OrderProcessorImpl) prepareProductInventoryMovement(item *entities.Orde
 		PreviousQuantity: float64(previousQuantity),
 		NewQuantity:      float64(updatedInventory.Quantity),
 		UnitCost:         item.UnitCost,
-		TotalCost:        float64(item.Quantity) * item.UnitCost,
+		TotalCost:        entities.RoundMoney(item.BillableQuantity() * item.UnitCost),
 		ReferenceType: func() *entities.InventoryMovementReferenceType {
 			t := entities.InventoryMovementReferenceTypePayment
 			return &t
@@ -1380,7 +1417,7 @@ type ingredientRecipeItem struct {
 
 // prepareIngredientRecipeItem prepares data for a single ingredient recipe without making database calls
 func (p *OrderProcessorImpl) prepareIngredientRecipeItem(ctx context.Context, recipe *entities.ProductRecipe, item *entities.OrderItem, order *entities.Order, payment *entities.Payment) (*ingredientRecipeItem, error) {
-	totalIngredientQuantity := recipe.Quantity * float64(item.Quantity)
+	totalIngredientQuantity := recipe.Quantity * item.BillableQuantity()
 
 	currentIngredient, err := p.ingredientRepo.GetByID(ctx, recipe.IngredientID, order.OrganizationID)
 	if err != nil {
@@ -1478,7 +1515,7 @@ func (p *OrderProcessorImpl) prepareRefundProductInventoryMovement(item *entitie
 		PreviousQuantity: float64(previousQuantity),
 		NewQuantity:      float64(updatedInventory.Quantity),
 		UnitCost:         item.UnitCost,
-		TotalCost:        float64(refundedQuantity) * item.UnitCost,
+		TotalCost:        entities.RoundMoney(item.BillableQuantityFor(refundedQuantity) * item.UnitCost),
 		ReferenceType: func() *entities.InventoryMovementReferenceType {
 			t := entities.InventoryMovementReferenceTypeRefund
 			return &t
@@ -1544,7 +1581,7 @@ func (p *OrderProcessorImpl) prepareRefundedIngredientRecipeData(ctx context.Con
 // prepareRefundedIngredientRecipeItem prepares data for a single ingredient recipe restoration
 func (p *OrderProcessorImpl) prepareRefundedIngredientRecipeItem(ctx context.Context, recipe *entities.ProductRecipe, item *entities.OrderItem, order *entities.Order, payment *entities.Payment, refundRatio float64, reason string, refundedBy uuid.UUID, refundAmount float64) (*ingredientRecipeItem, error) {
 	// Calculate total ingredient quantity needed based on order item quantity
-	totalIngredientQuantity := recipe.Quantity * float64(item.Quantity)
+	totalIngredientQuantity := recipe.Quantity * item.BillableQuantity()
 
 	// Get current ingredient details
 	currentIngredient, err := p.ingredientRepo.GetByID(ctx, recipe.IngredientID, order.OrganizationID)