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/tests/integration/test_products.py

@@ -30,6 +30,8 @@ from app.models import OperationLog
 pytestmark = pytest.mark.integration
 
 _PRODUCTS = "/api/v1/base/products"
+_CUSTOMERS = "/api/v1/base/customers"
+_BUSINESSES = "/api/v1/business/businesses"
 
 
 def _pname() -> str:
@@ -67,6 +69,48 @@ def _logs_of(username: str) -> list[OperationLog]:
         )
 
 
+@pytest.fixture
+def make_business_for_product(client: TestClient):
+    """造「客户 → 业务 → 明细挂指定产品」的最小引用链（删除保护场景）。
+
+    business_items.product_id 是 RESTRICT 外键，产品删除保护以此为引用源；
+    客户/业务均按 t_ 前缀与系统编号由 conftest 兜底清理。
+    """
+
+    def _factory(headers: dict, product_id: int) -> dict:
+        customer = client.post(
+            _CUSTOMERS,
+            json={
+                "company_name": f"t_引用公司_{uuid.uuid4().hex[:8]}",
+                "owner_name": "张老板",
+                "customer_type": "personal",
+            },
+            headers=headers,
+        )
+        assert customer.status_code == 200, customer.text
+        business = client.post(
+            _BUSINESSES,
+            json={
+                "customer_id": customer.json()["data"]["id"],
+                "business_date": "2026-01-01",
+                "items": [
+                    {
+                        "product_id": product_id,
+                        "product_name": f"t_引用产品_{uuid.uuid4().hex[:6]}",
+                        "product_category": PRODUCT_CATEGORY_MUNICIPAL_WATER,
+                        "quantity": "1.00",
+                        "sale_price": "10.00",
+                    }
+                ],
+            },
+            headers=headers,
+        )
+        assert business.status_code == 200, business.text
+        return business.json()["data"]
+
+    return _factory
+
+
 class TestAuth:
     """鉴权：登录后接口，裸访问一律 401。"""
 
@@ -120,6 +164,58 @@ class TestCrudFlow:
         assert "产品不存在" in detail.json()["message"]
 
 
+class TestDeleteProtection:
+    """删除保护：被业务/发货明细引用的产品禁删（customer 先例同构）。"""
+
+    def test_被业务明细引用_400且产品仍在(
+        self, client: TestClient, normal: dict, make_business_for_product
+    ) -> None:
+        product = _create(client, normal["headers"])
+        business = make_business_for_product(normal["headers"], product["id"])
+        resp = client.delete(f"{_PRODUCTS}/{product['id']}", headers=normal["headers"])
+        assert resp.status_code == 400, resp.text
+        assert "引用" in resp.json()["message"]
+        # 产品仍在（引导改走停用而非物理删除）
+        still = client.get(f"{_PRODUCTS}/{product['id']}", headers=normal["headers"])
+        assert still.status_code == 200
+        # 业务明细行确实引用了该产品（护栏：确认保护不是空转）
+        detail = client.get(f"{_BUSINESSES}/{business['id']}", headers=normal["headers"]).json()[
+            "data"
+        ]
+        assert detail["items"][0]["product_id"] == product["id"]
+
+    def test_被发货明细引用_400(
+        self, client: TestClient, normal: dict, make_business_for_product
+    ) -> None:
+        product = _create(client, normal["headers"])
+        make_business_for_product(normal["headers"], product["id"])
+        # 在业务明细引用之外再挂一笔发货明细（同样引用 products.id）
+        businesses = client.get("/api/v1/business/businesses", headers=normal["headers"]).json()[
+            "data"
+        ]
+        business_id = businesses["list"][0]["id"]
+        shipment = client.post(
+            "/api/v1/business/shipments",
+            json={
+                "business_id": business_id,
+                "ship_date": "2026-01-01",
+                "items": [
+                    {
+                        "product_id": product["id"],
+                        "product_name": product["product_name"],
+                        "quantity": "1.00",
+                        "sale_price": "10.00",
+                    }
+                ],
+            },
+            headers=normal["headers"],
+        )
+        assert shipment.status_code == 200, shipment.text
+        # 此前业务明细引用已令删除被拒，此处再证发货引用同一防线
+        resp = client.delete(f"{_PRODUCTS}/{product['id']}", headers=normal["headers"])
+        assert resp.status_code == 400, resp.text
+
+
 class TestValidationRules:
     """类别值域与组合唯一 —— 核算阈值锚点不脏、业务标识不重复。"""