feat: 个人用户聚合视图 — 客户管理双Tab+四模块数据聚合

- customers 表新增 customer_type 列 (unit/individual)
- 客户管理页：单位客户 | 个人用户 Tab 切换
- 个人用户 Tab: 直接内嵌4 Tab 显示所有个人用户的聚合数据
- 个人用户通过业务模块快速新建产生 (customer_type=individual)
- 4个业务API新增 customer_type 筛选参数（JOIN customers）
- 客户列表排序: 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()