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.go

@@ -233,6 +233,39 @@ func (h *ProductHandler) Detail(c *gin.Context) {
 	util.RespondSuccess(c, product)
 }
 
+// PriceHistory GET /api/v1/products/:id/price-history
+// 进价变更历史：从已审核入库单的明细单价按时间派生（零额外表）。
+// 返回按日期倒序的 {date, price} 点；前端据相邻点算涨跌。
+func (h *ProductHandler) PriceHistory(c *gin.Context) {
+	shopID := middleware.GetShopID(c)
+	id := c.Param("id")
+
+	type pricePoint struct {
+		Date  string  `json:"date"`
+		Price float64 `json:"price"`
+	}
+	points := make([]pricePoint, 0)
+	// 守多租户：order 的 shop_id 必须等于当前店；只取已审核单的正单价。
+	h.db.Raw(`
+		SELECT o.order_date AS date, sii.unit_price AS price
+		FROM stock_in_items sii
+		JOIN stock_in_orders o ON o.id = sii.order_id
+		WHERE sii.product_id = ? AND o.shop_id = ? AND o.status = 'approved'
+		  AND sii.unit_price > 0
+		ORDER BY o.order_date DESC, sii.id DESC
+		LIMIT 20`, id, shopID).Scan(&points)
+
+	// 去掉相邻重复价（只保留价格发生变化的点）
+	deduped := make([]pricePoint, 0, len(points))
+	for i, p := range points {
+		if i > 0 && points[i-1].Price == p.Price {
+			continue
+		}
+		deduped = append(deduped, p)
+	}
+	util.RespondSuccess(c, deduped)
+}
+
 // QRCode GET /api/v1/products/:id/qrcode
 func (h *ProductHandler) QRCode(c *gin.Context) {
 	shopID := middleware.GetShopID(c)