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/receipt_service.py

@@ -284,8 +284,11 @@ def update_receipt(
         if receipt.settlement_status == SETTLEMENT_DONE:
             raise BadRequestError("已核算回款禁止修改")
         business = _ensure_business_writable(session, receipt.business_id, "修改")
-        # 返回值不取用：DTO 组装以 get_with_refs 重查为准（shipment 变更后同源）
-        _ensure_shipment_of_business(session, payload.shipment_id, receipt.business_id)
+        # 校验返回的 (shipment_no, ship_date) 即新批次 DTO 所需 —— shipment_id
+        # 已被赋为 payload 新值，与重查 get_with_refs 同源，省一次三表 join
+        shipment_no, ship_date = _ensure_shipment_of_business(
+            session, payload.shipment_id, receipt.business_id
+        )
 
         receipt.receipt_date = payload.receipt_date
         receipt.shipment_id = payload.shipment_id
@@ -293,7 +296,7 @@ def update_receipt(
         receipt.is_barter = payload.is_barter
         receipt.remark = payload.remark
         _recalc_totals(session, business)
-        out = _to_out(receipt_repository.get_with_refs(session, receipt_id))
+        out = _to_out((receipt, business.business_no, shipment_no, ship_date))
         batch_no = receipt.receipt_batch_no
         session.commit()
         # commit 后不再触碰 ORM 属性（audit_service 约定）：批次号已固化