fix(business): 删除保护补齐与单价小数位口径修复（e2e 发现）

- 客户删除保护：被业务引用 → 400（T4 占位注释在业务表就位后漏补，FK 500 暴露）
- 人员删除保护：业务归属（负责/协助）引用纳入检查（同样漏检 500）
- schema 单价类列（DECIMAL 14,4）小数位放行至 4 位：回显 '5000.0000'
  原样提交被两位校验永久拒收（编辑任何带精度单价的历史行都失败）
- 补 2 个集成用例锁定；文案同步「客户归属或业务归属引用」

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

backend/app/repositories/customer_repository.py

@@ -23,6 +23,19 @@ def get_by_id(session: Session, customer_id: int) -> Customer | None:
     return session.get(Customer, customer_id)
 
 
+def count_business_refs(session: Session, customer_id: int) -> int:
+    """被业务单据引用的次数（删除保护；业务表就位后补齐的检查）。"""
+    from app.models import Business
+
+    return int(
+        session.execute(
+            select(func.count())
+            .select_from(Business)
+            .where(Business.customer_id == customer_id)
+        ).scalar_one()
+    )
+
+
 def get_with_owner_name(session: Session, customer_id: int) -> Row | None:
     """查单个客户并带出归属人员姓名（详情、编辑回显与归属变更共用）。"""
     stmt = _base_select().where(Customer.id == customer_id)

backend/app/repositories/personnel_repository.py

@@ -94,6 +94,33 @@ def count_customer_ownerships(session: Session, personnel_ids: list[int]) -> dic
     return {int(owner_id): int(count) for owner_id, count in session.execute(stmt).all()}
 
 
+def count_business_roles_refs(session: Session, personnel_ids: list[int]) -> dict[int, int]:
+    """按人员批量统计业务单据归属引用（负责/协助合计；业务表就位后补齐的检查）。
+
+    businesses.owner/assistant_personnel_id 均为 RESTRICT 外键——漏检会让
+    「删除被业务引用的人员」以 500 暴露而非可读的 400。
+    """
+
+    from app.models import Business
+
+    owner = (
+        select(Business.owner_personnel_id, func.count(Business.id))
+        .where(Business.owner_personnel_id.in_(personnel_ids))
+        .group_by(Business.owner_personnel_id)
+    )
+    assistant = (
+        select(Business.assistant_personnel_id, func.count(Business.id))
+        .where(Business.assistant_personnel_id.in_(personnel_ids))
+        .group_by(Business.assistant_personnel_id)
+    )
+    refs: dict[int, int] = {}
+    for stmt in (owner, assistant):
+        for owner_id, count in session.execute(stmt).all():
+            if owner_id is not None:
+                refs[int(owner_id)] = refs.get(int(owner_id), 0) + int(count)
+    return refs
+
+
 def list_regions(session: Session, personnel_id: int) -> list[PersonnelRegion]:
     """查人员的全部辐射区域，按插入顺序返回。"""
     stmt = (