fix(backend): 修复库存 TOCTOU 竞态 + handler 白名单更新 + 对账接口

#29 ApproveStockOut：FOR UPDATE 锁定批次后再内存汇总总量，
     消除预检 SUM 与加锁之间的竞态窗口，减少一次 DB 往返

#30 partner/warehouse/user/product_attr/product_option Update 方法：
     - 绑定到独立 req struct，防止请求体覆盖记录 ID
     - 改用 db.Model.Where("shop_id=?").Updates(map) 白名单更新，
       数据库层强制 shop_id 隔离约束

#31 新增 GET /api/v1/admin/reconcile：对比 inventories 当前库存
     与 inventory_logs 流水净量，返回差异行，用于发现不平账异常

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

backend/internal/service/stock.go

@@ -163,12 +163,17 @@ func (s *StockService) ApproveStockOut(shopID, orderID, reviewerID uint64) error
 			productID := itemCopy.ProductID
 			needed := itemCopy.Quantity
 
-			// 1. 预检：SUM 是否充足
-			var totalQty float64
-			tx.Model(&model.Inventory{}).
-				Where("shop_id = ? AND warehouse_id = ? AND product_id = ? AND deleted_at IS NULL",
+			// 1. FOR UPDATE 锁定批次后再汇总，消除 TOCTOU 窗口
+			var batches []model.Inventory
+			tx.Set("gorm:query_option", "FOR UPDATE").
+				Where("shop_id = ? AND warehouse_id = ? AND product_id = ? AND quantity > 0 AND deleted_at IS NULL",
 					shopID, warehouseID, productID).
-				Select("COALESCE(SUM(quantity), 0)").Scan(&totalQty)
+				Order("created_at ASC").Find(&batches)
+
+			var totalQty float64
+			for _, b := range batches {
+				totalQty += b.Quantity
+			}
 			if totalQty < needed {
 				return fmt.Errorf("%w: product_id=%d, available=%.3f, required=%.3f",
 					ErrInsufficientStock, productID, totalQty, needed)
@@ -176,13 +181,7 @@ func (s *StockService) ApproveStockOut(shopID, orderID, reviewerID uint64) error
 
 			qtyBefore := totalQty
 
-			// 2. FIFO 扣减批次
-			var batches []model.Inventory
-			tx.Set("gorm:query_option", "FOR UPDATE").
-				Where("shop_id = ? AND warehouse_id = ? AND product_id = ? AND quantity > 0 AND deleted_at IS NULL",
-					shopID, warehouseID, productID).
-				Order("created_at ASC").Find(&batches)
-
+			// 2. FIFO 扣减
 			remaining := needed
 			for i := range batches {
 				if remaining <= 0 {