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,

client/lib/models/inventory.dart

@@ -15,7 +15,8 @@ class Inventory {
   final String spec;
   final String unit;
   final String warehouseName;
-  final double? unitPrice;
+  final double? unitPrice; // 成本价（入库进价）
+  final double? salePrice; // 单价（product 售价）
   final String? productionDate;
   final String batchNo;
   final String supplierName;
@@ -37,6 +38,7 @@ class Inventory {
     this.unit = '',
     this.warehouseName = '',
     this.unitPrice,
+    this.salePrice,
     this.productionDate,
     this.batchNo = '',
     this.supplierName = '',
@@ -59,6 +61,7 @@ class Inventory {
         unit: json['unit'] as String? ?? '',
         warehouseName: json['warehouse_name'] as String? ?? '',
         unitPrice: (json['unit_price'] as num?)?.toDouble(),
+        salePrice: (json['sale_price'] as num?)?.toDouble(),
         productionDate: _parseDate(json['production_date'] as String?),
         batchNo: json['batch_no'] as String? ?? '',
         supplierName: json['supplier_name'] as String? ?? '',