feat: 客户管理新增个人用户类型，双Tab分离管理

- customers 表新增 customer_type 列 (unit/individual)
- 列表按个人优先排序，支持 customer_type 筛选
- PC端 el-radio-group 切换：单位客户 | 个人用户
- 个人用户表单: 仅姓名+备注，隐藏行业/地址/业务/月费/客户经理/联系人
- 所有业务模块客户搜索下拉中个人用户置顶

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

backend/app/api/customers.py

@@ -50,6 +50,7 @@ async def list_customers(
     industry: Optional[str] = Query(None),
     service: Optional[str] = Query(None),
     manager_id: Optional[str] = Query(None),
+    customer_type: Optional[str] = Query(None),
     page: int = Query(1, ge=1),
     page_size: int = Query(100, ge=1, le=1000),
     current_user: dict = Depends(get_current_user),
@@ -60,6 +61,8 @@ async def list_customers(
 
     base_query = select(Customer)
 
+    if customer_type:
+        base_query = base_query.where(Customer.customer_type == customer_type)
     if industry:
         base_query = base_query.where(Customer.industry.ilike(f"%{industry}%"))
     if service:
@@ -89,7 +92,7 @@ async def list_customers(
 
     # Paginate
     offset = (page - 1) * page_size
-    query = base_query.order_by(Customer.name).offset(offset).limit(page_size)
+    query = base_query.order_by(Customer.customer_type, Customer.name).offset(offset).limit(page_size)
     result = await db.execute(query)
     items = result.scalars().all()