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_weight_test.go

@@ -0,0 +1,106 @@
+package processor
+
+import (
+	"testing"
+
+	"apskel-pos-be/internal/constants"
+	"apskel-pos-be/internal/entities"
+)
+
+func fPtr(v float64) *float64 { return &v }
+
+func TestResolveLineQuantity(t *testing.T) {
+	weightProduct := &entities.Product{Name: "Ikan Tude", SellBy: constants.SellByWeight}
+	unitProduct := &entities.Product{Name: "Nasi Goreng", SellBy: constants.SellByUnit}
+
+	tests := []struct {
+		name         string
+		product      *entities.Product
+		quantity     int
+		weight       *float64
+		wantQuantity int
+		wantWeight   *float64
+		wantErr      bool
+	}{
+		{
+			name: "weighed line is pinned to quantity 1",
+			// Mirrors chk_order_items_weight_single_line: even if a client sends 3,
+			// one weighing is one line.
+			product: weightProduct, quantity: 3, weight: fPtr(4.2),
+			wantQuantity: 1, wantWeight: fPtr(4.2),
+		},
+		{
+			name:    "weight is rounded to stored precision",
+			product: weightProduct, quantity: 1, weight: fPtr(5.6004),
+			wantQuantity: 1, wantWeight: fPtr(5.6),
+		},
+		{
+			name:    "weight product without a weight is rejected",
+			product: weightProduct, quantity: 1, weight: nil,
+			wantErr: true,
+		},
+		{
+			name:    "weight that rounds away to zero is rejected",
+			product: weightProduct, quantity: 1, weight: fPtr(0.0004),
+			wantErr: true,
+		},
+		{
+			name:    "unit line keeps its quantity and carries no weight",
+			product: unitProduct, quantity: 3, weight: nil,
+			wantQuantity: 3, wantWeight: nil,
+		},
+		{
+			name: "weight on a unit product is rejected",
+			// Otherwise the line would silently be priced by weight.
+			product: unitProduct, quantity: 1, weight: fPtr(4.2),
+			wantErr: true,
+		},
+		{
+			name:    "non-positive quantity on a unit product is rejected",
+			product: unitProduct, quantity: 0, weight: nil,
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotQty, gotWeight, err := resolveLineQuantity(tt.product, tt.quantity, tt.weight)
+
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected an error, got quantity=%v weight=%v", gotQty, gotWeight)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if gotQty != tt.wantQuantity {
+				t.Errorf("quantity = %v, want %v", gotQty, tt.wantQuantity)
+			}
+			switch {
+			case tt.wantWeight == nil && gotWeight != nil:
+				t.Errorf("weight = %v, want nil", *gotWeight)
+			case tt.wantWeight != nil && gotWeight == nil:
+				t.Errorf("weight = nil, want %v", *tt.wantWeight)
+			case tt.wantWeight != nil && *gotWeight != *tt.wantWeight:
+				t.Errorf("weight = %v, want %v", *gotWeight, *tt.wantWeight)
+			}
+		})
+	}
+}
+
+// A product created before this feature has sell_by defaulted to 'unit' by the
+// migration, but a zero-valued struct must behave the same way.
+func TestResolveLineQuantityTreatsEmptySellByAsUnit(t *testing.T) {
+	legacy := &entities.Product{Name: "Legacy"}
+
+	quantity, weight, err := resolveLineQuantity(legacy, 2, nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if quantity != 2 || weight != nil {
+		t.Errorf("got quantity=%v weight=%v, want quantity=2 weight=nil", quantity, weight)
+	}
+}