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

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? ?? '',