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/models/analytics.go

@@ -233,6 +233,7 @@ type ProductAnalyticsData struct {
 	CategoryName            string    `json:"category_name"`
 	CategoryOrder           int       `json:"category_order"`
 	QuantitySold            int64     `json:"quantity_sold"`
+	WeightSold              float64   `json:"weight_sold"`
 	Revenue                 float64   `json:"revenue"`
 	AveragePrice            float64   `json:"average_price"`
 	OrderCount              int64     `json:"order_count"`
@@ -359,6 +360,7 @@ type ParentCategoryAnalyticsProductData struct {
 	ProductSku              string    `json:"product_sku"`
 	ProductPrice            float64   `json:"product_price"`
 	QuantitySold            int64     `json:"quantity_sold"`
+	WeightSold              float64   `json:"weight_sold"`
 	Revenue                 float64   `json:"revenue"`
 	AveragePrice            float64   `json:"average_price"`
 	OrderCount              int64     `json:"order_count"`

internal/models/order.go

@@ -44,6 +44,8 @@ type OrderItem struct {
 	ProductID           uuid.UUID
 	ProductVariantID    *uuid.UUID
 	Quantity            int
+	Weight              *float64
+	UnitID              *uuid.UUID
 	UnitPrice           float64
 	TotalPrice          float64
 	UnitCost            float64
@@ -99,7 +101,8 @@ type CreateOrderItemRequest struct {
 	ProductID        uuid.UUID                `validate:"required"`
 	ProductVariantID *uuid.UUID               `validate:"omitempty"`
 	Quantity         int                      `validate:"required,min=1"`
-	UnitPrice        *float64                 `validate:"omitempty,min=0"` // Optional, will use database price if not provided
+	Weight           *float64                 `validate:"omitempty,gt=0"`
+	UnitPrice        *float64                 `validate:"omitempty,min=0"` // Ignored: price always comes from the database
 	Modifiers        []map[string]interface{} `validate:"omitempty"`
 	Notes            *string                  `validate:"omitempty,max=500"`
 	Metadata         map[string]interface{}   `validate:"omitempty"`
@@ -191,6 +194,10 @@ type OrderItemResponse struct {
 	CategoryID          *uuid.UUID
 	CategoryName        *string
 	Quantity            int
+	Weight              *float64
+	UnitID              *uuid.UUID
+	UnitName            *string
+	UnitAbbreviation    *string
 	UnitPrice           float64
 	TotalPrice          float64
 	UnitCost            float64

internal/models/product.go

@@ -19,6 +19,7 @@ type Product struct {
 	BusinessType   constants.BusinessType
 	ImageURL       *string
 	PrinterType    string
+	SellBy         string
 	UnitID         *uuid.UUID
 	HasIngredients bool
 	Metadata       map[string]interface{}
@@ -52,6 +53,7 @@ type CreateProductRequest struct {
 	PrinterType    *string                `validate:"omitempty,max=50"`
 	PrintToChecker *bool                  `validate:"omitempty"`
 	UnitID         *uuid.UUID             `validate:"omitempty"`
+	SellBy         string                 `validate:"omitempty,oneof=unit weight"`
 	HasIngredients bool                   `validate:"omitempty"`
 	Metadata       map[string]interface{}
 	Variants       []CreateProductVariantRequest `validate:"omitempty,dive"`
@@ -73,6 +75,7 @@ type UpdateProductRequest struct {
 	PrinterType    *string    `validate:"omitempty,max=50"`
 	PrintToChecker *bool      `validate:"omitempty"`
 	UnitID         *uuid.UUID `validate:"omitempty"`
+	SellBy         *string    `validate:"omitempty,oneof=unit weight"`
 	HasIngredients *bool      `validate:"omitempty"`
 	Metadata       map[string]interface{}
 	IsActive       *bool
@@ -110,6 +113,7 @@ type ProductResponse struct {
 	BusinessType   constants.BusinessType
 	ImageURL       *string
 	PrinterType    string
+	SellBy         string
 	PrintToChecker bool
 	UnitID         *uuid.UUID
 	HasIngredients bool