fix(product): 删除引用保护/单价小数位校验，IntegrityError 全局兜底转 400

- delete_product 补 count_refs 引用检查（业务明细+发货明细 RESTRICT
  外键）与并发窗口 IntegrityError→400 兜底（customer 先例同构），
  docstring 与两个 api 层过时注释同步更新
- 产品 schema 复用 schemas/business._check_amount：sale_price 走
  (PRICE_MAX, scale=4)、stock_quantity 走 AMOUNT_MAX——此前私写
  _parse_non_negative 不查小数位与容量，5 位小数被 Numeric(14,4)
  静默截断（舍入即算错钱）；新增 6 个秒级 schema 单测锁定口径
- 全局注册 IntegrityError handler：漏补引用保护的删除类操作收敛为
  400 通用文案（e2e 撞见一个补一个的模式无法保证全覆盖），完整
  异常记日志；rating/customer 的「非 1062 按 500」注释同步更正

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

backend/app/services/business_service.py

@@ -380,16 +380,17 @@ def update_business(
     ):
         if row is None:
             raise NotFoundError("业务不存在")
-        business, company_name, owner_name, assistant_name = row
+        business, _, owner_name, assistant_name = row
 
         # 终态门槛与发货/回款写准入同口径：明细与汇总是 M3 核算基线，
         # 已完成/已终止的业务禁改（发货/回款服务以同一理由禁写终态业务）
         if business.status != BUSINESS_STATUS_ACTIVE:
             raise BadRequestError("业务已完成或已终止，禁止编辑业务")
 
-        if payload.customer_id != business.customer_id:
-            if business_repository.has_docs(session, business_id):
-                raise BadRequestError("业务已有发货或回款记录，不能更换客户")
+        if payload.customer_id != business.customer_id and business_repository.has_docs(
+            session, business_id
+        ):
+            raise BadRequestError("业务已有发货或回款记录，不能更换客户")
         customer = _ensure_customer(session, payload.customer_id)
         if payload.owner_personnel_id is not None:
             _ensure_personnel_active(session, payload.owner_personnel_id, "负责")