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/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")  # unit / individual
     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="")