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/router/router.go

@@ -117,6 +117,7 @@ func Setup(r *gin.Engine, db *gorm.DB) {
 			products.POST("", productH.Create)
 			products.POST("/find-or-create", productH.FindOrCreate)
 			products.GET("/:id/detail", productH.Detail)
+			products.GET("/:id/price-history", productH.PriceHistory)
 			products.GET("/:id/qrcode", productH.QRCode)
 			products.PUT("/:id", productH.Update)
 			products.DELETE("/:id", productH.Delete)