feat(backend): 库存接口带出售价 sale_price（库存屏成本价/单价两列地基）

照原型库存表「成本价(进价)+单价(售价)」两列：
- 库存查询 SELECT 加 p.sale_price AS sale_price；成本价 unit_price 补 p.purchase_price 兜底
- inventoryRow 加 SalePrice；前端 Inventory model 加 salePrice
- 多租户隔离不变（baseWhere 仍 inv.shop_id）；库存测试通过

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

backend/internal/handler/inventory.go

@@ -36,7 +36,8 @@ type inventoryRow struct {
 	Spec           string   `json:"spec"`
 	Unit           string   `json:"unit"`
 	WarehouseName  string   `json:"warehouse_name"`
-	UnitPrice      *float64 `json:"unit_price"`
+	UnitPrice      *float64 `json:"unit_price"` // 成本价（入库进价）
+	SalePrice      *float64 `json:"sale_price"` // 单价（product 售价）
 	ProductionDate *string  `json:"production_date"`
 	BatchNo        string   `json:"batch_no"`
 	SupplierName   string   `json:"supplier_name"`
@@ -131,7 +132,8 @@ func (h *InventoryHandler) List(c *gin.Context) {
 		  COALESCE(NULLIF(p.spec,''), NULLIF(sii.spec,''), inv.spec, '') AS spec,
 		  COALESCE(NULLIF(p.unit,''), inv.unit, '') AS unit,
 		  COALESCE(NULLIF(w.name,''), inv.warehouse_name, '') AS warehouse_name,
-		  COALESCE(sii.unit_price, inv.unit_price) AS unit_price,
+		  COALESCE(sii.unit_price, inv.unit_price, p.purchase_price) AS unit_price,
+		  p.sale_price AS sale_price,
 		  COALESCE(DATE(sii.production_date), DATE(inv.production_date)) AS production_date,
 		  COALESCE(NULLIF(sii.batch_no,''), inv.batch_no, '') AS batch_no,
 		  inv.supplier_name,