perf: 系统级性能优化与并发安全修复

## 并发安全修复 (4处)
- scrap.py: 报废执行添加 SELECT FOR UPDATE 悲观锁，消除 TOCTOU 竞态
- stock.py (adjust_stock): 盘点调整添加 for_update=True 行锁
- outbound_service.py: 低库存预警 SMTP 调用移到 commit 之后，避免长事务
- trans_service.py: execute_dispatch 按 (source_table, id) 排序 items，消除死锁风险

## N+1 查询优化 (2处)
- inventory_task.py: _prefetch_inventory_map 单条 UNION ALL+GROUP BY 替代循环内逐条查询（N*4次→2次）
- stock.py (export_stocktake): get_borrowed_qty 批量 GROUP BY 替代逐条 TransBorrow 查询（~18000次→1次）

## BOM 列表性能重构
- bom_service.py: get_bom_list 单条 GROUP BY+string_agg+分页，消除 N+1 循环查询
- bom_service.py: 新增 get_bom_summary (轻量 GROUP BY category+COUNT)
- bom.py: 新增 /api/v1/bom/summary 路由，/list 支持 category 过滤

## Odoo 基础信息懒加载
- base_service.py: 新增 get_odoo_summary (GROUP BY category+COUNT)
- base.py: 新增 /api/v1/inbound/base/odoo-summary 路由
- buyOdoo.vue: 懒加载分组架构 (fetchOdooSummary + loadGroupItems)
- material_base.ts: 新增 getOdooSummary API

## 前端 Bug 修复
- BomManage.vue: 懒加载分组 (fetchBomSummary + loadGroupItems + collapse)
- BomManage.vue: 适配新 API 格式 (res.data.items 替代 res.data)
- buyOdoo.vue: 移除 "点击展开加载" 文字
- Selection.vue + borrow/apply/index.vue: openBomSelect 适配新 API 格式

inventory-backend/app/api/v1/scrap.py

@@ -261,19 +261,19 @@ class ScrapService:
                 created_records.append(scrap_record)
                 continue
 
-            # 获取库存记录
+            # 获取库存记录 — ★ 修复并发：使用悲观锁防止超卖/负库存
             stock_record = None
             if source_table == 'stock_product':
-                stock_record = StockProduct.query.get(stock_id)
+                stock_record = StockProduct.query.with_for_update().get(stock_id)
             elif source_table == 'stock_semi':
-                stock_record = StockSemi.query.get(stock_id)
+                stock_record = StockSemi.query.with_for_update().get(stock_id)
             elif source_table == 'stock_buy':
-                stock_record = StockBuy.query.get(stock_id)
+                stock_record = StockBuy.query.with_for_update().get(stock_id)
 
             if not stock_record:
                 raise ValueError(f'库存记录不存在: ID={stock_id}')
 
-            # 检查可用数量
+            # 检查可用数量（锁已持有，TOCTOU 窗口已消除）
             avail_qty = float(stock_record.available_quantity) if stock_record.available_quantity else 0
             if avail_qty < scrap_qty:
                 raise ValueError(f"SKU {stock_record.sku} 可用库存不足，当前可用: {avail_qty}")