refactor(business): 编辑出参取校验返回值省重查，submit_change 批量预载

- update_receipt 取 _ensure_shipment_of_business 返回值组装出参
  （shipment_id 已赋为 payload 新值，与 get_with_refs 重查同源），
  省一次三表 join；update_shipment 同理取锁定行的 business_no
- 删 business_repository.count_items（无任何调用的死代码，删除保护
  实际走 has_docs）
- submit_change 校验段定义与岗位各一条 IN 批量预载（逐条查询在批次
  扩大时是写侧 N+1）；岗位字典键 casefold 归一保持与单条版 ci 校对
  等价的命中语义；旧值留痕保持逐条（窗口函数优化留待 M3 一并设计）

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

backend/app/services/shipment_service.py

@@ -317,7 +317,8 @@ def update_shipment(
         total = shipment_repository.sum_item_amount(session, shipment_id)
         shipment.total_amount = total if total is not None else Decimal("0")
         _recalc_totals(session, business)
-        out = _to_out(shipment_repository.get_with_business_no(session, shipment_id))
+        # business_id 创建时定死不可改挂，锁定的 business.business_no 与重查同源
+        out = _to_out((shipment, business.business_no))
         shipment_no = shipment.shipment_no
         session.commit()
         # commit 后不再触碰 ORM 属性（audit_service 约定）：单号已固化