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/visits.py

@@ -62,6 +62,7 @@ async def list_visits(
     date_from: Optional[str] = Query(None),
     date_to: Optional[str] = Query(None),
     customer_id: Optional[str] = Query(None),
+    customer_type: Optional[str] = Query(None),
     current_user: dict = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
@@ -77,6 +78,8 @@ async def list_visits(
         query = query.where(Visit.visit_date <= parse_date(date_to))
     if customer_id:
         query = query.where(Visit.customer_id == uuid.UUID(customer_id))
+    if customer_type:
+        query = query.join(Customer, Visit.customer_id == Customer.id).where(Customer.customer_type == customer_type)
 
     query = query.order_by(Visit.visit_date.desc(), Visit.created_at.desc()).limit(200)
     result = await db.execute(query)