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/models/customer.py

@@ -11,6 +11,7 @@ class Customer(Base):
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     name: Mapped[str] = mapped_column(String(200), index=True)
+    customer_type: Mapped[str] = mapped_column(String(20), default="unit")
     industry: Mapped[str] = mapped_column(String(100), default="")
     address: Mapped[str] = mapped_column(String(500), default="")
     in_use_services: Mapped[str] = mapped_column(Text, default="")