feat(backend): 商品价格历史接口（从入库单价派生）

GET /products/:id/price-history：从已审核入库单明细 unit_price 按 order_date
倒序派生进价变更点，去相邻重复价，守多租户 shop_id（零额外表）。
新增测试 PriceHistory（含草稿排除 + 倒序断言）。go build/vet/test 全过。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TSKEiHsvauyxYUW2itzUXX

backend/internal/handler/product_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -266,3 +267,41 @@ func TestProductHandler_UniqueShopCode(t *testing.T) {
 	require.Error(t, err)
 	assert.True(t, errors.Is(err, gorm.ErrDuplicatedKey))
 }
+
+func TestProductHandler_PriceHistory(t *testing.T) {
+	db := testutil.SetupTestDB()
+	shop := testutil.CreateTestShop(db, "PH001")
+	user := testutil.CreateTestUser(db, shop.ID, "admin", "pass", "admin")
+	wh := testutil.CreateTestWarehouse(db, shop.ID, "W")
+	product := testutil.CreateTestProduct(db, shop.ID, "Maotai")
+	token := getAuthToken(user.ID, shop.ID, "admin")
+	r := setupProtectedRouter(db)
+
+	mkOrder := func(no string, daysAgo int, price float64, status string) {
+		o := &model.StockInOrder{
+			TenantBase:  model.TenantBase{ShopID: shop.ID},
+			OrderNo:     no,
+			WarehouseID: wh.ID,
+			OperatorID:  user.ID,
+			Status:      status,
+			OrderDate:   model.Date{Time: time.Now().AddDate(0, 0, -daysAgo)},
+		}
+		require.NoError(t, db.Create(o).Error)
+		require.NoError(t, db.Create(&model.StockInItem{
+			OrderID: o.ID, ShopID: shop.ID, ProductID: product.ID,
+			UnitPrice: price, Quantity: 1,
+		}).Error)
+	}
+	mkOrder("RK-PH-1", 60, 2580, "approved")
+	mkOrder("RK-PH-2", 30, 2620, "approved")
+	mkOrder("RK-PH-3", 5, 2680, "approved")
+	mkOrder("RK-PH-4", 1, 9999, "draft") // 草稿不计入
+
+	w := makeRequest(r, "GET",
+		fmt.Sprintf("/api/v1/products/%d/price-history", product.ID), token, nil)
+	assert.Equal(t, http.StatusOK, w.Code)
+	data := parseResponse(w)["data"].([]interface{})
+	assert.Len(t, data, 3) // 三个已审核不同价；草稿排除
+	// 倒序：最新价在前
+	assert.Equal(t, float64(2680), data[0].(map[string]interface{})["price"])
+}