fix(business): 编辑/流转/归属加锁与终态编辑门槛，has_docs 单点收口

- get_with_names 加 for_update 参数：update/change_status/change_ownership
  以锁定读取行，与发货/回款写路径同一锁口径——「有发货禁换客户」的
  快照读校验不再被并发新增发货旁路（此前 delete 有锁、唯独编辑无锁）
- update_business 补终态门槛（明细/汇总是 M3 核算基线，与发货/回款
  服务禁写终态同口径）；前端编辑按钮按终态禁用并加 tooltip
- 新增 business_repository.has_docs（单条 OR exists）：替换 update
  换客户与 delete 两处内联双查询，M3 新增挂靠单据类型时只改一处
- 出参组装复用首查 row：change_ownership 的姓名侧按「变更项取校验
  函数返回的新人名、未变项沿用首查名」处理（首查名是变更前快照）
- 新增终态编辑 400 且明细未动的集成用例

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

backend/app/repositories/business_repository.py

@@ -7,7 +7,7 @@ models/customer.py 定义）。
 
 from datetime import date
 
-from sqlalchemy import Row, Select, delete, func, or_, select
+from sqlalchemy import Row, Select, delete, exists, func, or_, select
 from sqlalchemy.orm import Session, aliased
 
 from app.models import (
@@ -17,18 +17,35 @@ from app.models import (
     Customer,
     OwnershipChangeLog,
     Personnel,
+    Receipt,
+    Shipment,
 )
 from app.models.customer import OWNER_TYPE_BUSINESS
 
 
 def get_by_id(session: Session, business_id: int, *, for_update: bool = False) -> Business | None:
-    """按主键取业务行；for_update 供删除保护与状态机使用。"""
+    """按主键取业务行；for_update 供删除保护、编辑门槛与状态机使用。"""
     stmt = select(Business).where(Business.id == business_id)
     if for_update:
         stmt = stmt.with_for_update()
     return session.execute(stmt).scalars().first()
 
 
+def has_docs(session: Session, business_id: int) -> bool:
+    """业务名下是否已有发货单或回款（换客户/删除保护的引用检查）。
+
+    单条 OR exists 一次往返；新增挂靠业务的单据类型时在此补充条件，
+    编辑与删除两处共用同一判定，防「delete 补了检查、update 漏补」的分叉。
+    """
+    stmt = select(
+        or_(
+            exists().where(Shipment.business_id == business_id),
+            exists().where(Receipt.business_id == business_id),
+        )
+    )
+    return bool(session.execute(stmt).scalar())
+
+
 def create(session: Session, business: Business) -> None:
     """新增业务行（不 commit，flush 让自增 id 就位）。"""
     session.add(business)
@@ -36,19 +53,26 @@ def create(session: Session, business: Business) -> None:
 
 
 def get_with_names(
-    session: Session, business_id: int
+    session: Session, business_id: int, *, for_update: bool = False
 ) -> tuple[Business, str, str | None, str | None] | None:
-    """业务行 + 客户名 + 负责人名 + 协助人名（DTO 与审计摘要共用）。"""
+    """业务行 + 客户名 + 负责人名 + 协助人名（DTO 与审计摘要共用）。
+
+    for_update 供编辑/状态流转/归属变更使用：与发货/回款写路径的
+    get_by_id(for_update=True) 同一锁口径，编辑事务与单据写事务互斥，
+    「有发货禁换客户」的快照读校验不再被并发新增旁路。
+    """
     owner = aliased(Personnel)
     assistant = aliased(Personnel)
-    row = session.execute(
+    stmt = (
         select(Business, Customer.company_name, owner.personnel_name, assistant.personnel_name)
         .join(Customer, Customer.id == Business.customer_id)
         .outerjoin(owner, owner.id == Business.owner_personnel_id)
         .outerjoin(assistant, assistant.id == Business.assistant_personnel_id)
         .where(Business.id == business_id)
-    ).first()
-    return row
+    )
+    if for_update:
+        stmt = stmt.with_for_update()
+    return session.execute(stmt).first()
 
 
 def _apply_filters(