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/repositories/inventory_repository.dart

@@ -47,7 +47,7 @@ class InventoryRepository {
     }
   }
 
-  Future<PageResult<BatchRecord>> listBatches({
+  Future<PageResult<ProductTrackingRecord>> listProducts({
     int? productId,
     int? warehouseId,
     int page = 1,
@@ -60,14 +60,14 @@ class InventoryRepository {
         if (productId != null) 'product_id': productId,
         if (warehouseId != null) 'warehouse_id': warehouseId,
       };
-      final resp = await _client.get('/inventory/batches', params: params);
+      final resp = await _client.get('/inventory/products', params: params);
       return PageResult.fromJson(
         resp.data as Map<String, dynamic>,
-        BatchRecord.fromJson,
+        ProductTrackingRecord.fromJson,
       );
     } on DioException catch (e) {
       throw AppException(
-        e.response?.data?['error'] as String? ?? '获取批次数据失败',
+        e.response?.data?['error'] as String? ?? '获取商品追踪数据失败',
         statusCode: e.response?.statusCode,
       );
     }