feat: 商品追踪页面 + 出库提交库存校验

后端：
- feat(backend): 重命名 Batches→Products 接口，新增库存状态(在售/已卖出)和买家信息
- feat(backend): 出库单创建/提交时校验仓库库存，不足则返回明确错误信息
- fix(backend): 出库创建 status 判断逻辑修复（空值默认 draft）

前端：
- feat(client): 批次追踪改为商品追踪，新增状态列(在售/已卖出)和买家/时间列
- fix(client): 无批次号时显示"无批次"而非空
- refactor(client): BatchRecord → ProductTrackingRecord，repository 接口更新

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/models/inventory.dart

@@ -43,8 +43,8 @@ class Inventory {
   }
 }
 
-// 批次追踪：已审核入库单的明细行
-class BatchRecord {
+// 商品追踪：已审核入库单的明细行，含库存状态和买家信息
+class ProductTrackingRecord {
   final int id;
   final int productId;
   final String? productName;
@@ -58,8 +58,13 @@ class BatchRecord {
   final String? orderDate;
   final String? warehouseName;
   final String? supplierName;
+  // 库存状态
+  final double currentQty;
+  final String status; // in_stock | sold_out
+  final String? buyerName;
+  final String? soldAt;
 
-  const BatchRecord({
+  const ProductTrackingRecord({
     required this.id,
     required this.productId,
     this.productName,
@@ -73,14 +78,20 @@ class BatchRecord {
     this.orderDate,
     this.warehouseName,
     this.supplierName,
+    required this.currentQty,
+    required this.status,
+    this.buyerName,
+    this.soldAt,
   });
 
-  factory BatchRecord.fromJson(Map<String, dynamic> json) {
+  bool get isSoldOut => status == 'sold_out';
+
+  factory ProductTrackingRecord.fromJson(Map<String, dynamic> json) {
     final product = json['product'] as Map<String, dynamic>?;
     final order = json['order'] as Map<String, dynamic>?;
     final warehouse = order?['warehouse'] as Map<String, dynamic>?;
     final partner = order?['partner'] as Map<String, dynamic>?;
-    return BatchRecord(
+    return ProductTrackingRecord(
       id: (json['id'] as num).toInt(),
       productId: (json['product_id'] as num).toInt(),
       productName: product?['name'] as String?,
@@ -94,6 +105,12 @@ class BatchRecord {
       orderDate: order?['order_date'] as String?,
       warehouseName: warehouse?['name'] as String?,
       supplierName: partner?['name'] as String?,
+      currentQty: json['current_qty'] != null
+          ? (json['current_qty'] as num).toDouble()
+          : 0,
+      status: json['status'] as String? ?? 'in_stock',
+      buyerName: json['buyer_name'] as String?,
+      soldAt: json['sold_at'] as String?,
     );
   }
 }