refactor: 权限组简化为2组(readonly/write) + Transfer穿梭框选择数据库

- access_type 从3值(readonly/datachange/ddl)简化为2值(readonly/write)
- readonly=只读查看(关联只读/从库用于查询) write=写入变更(关联主库用于工单)
- 前端InstanceGroupPage重写: 2权限类型卡片+自定义Transfer穿梭框组件
- Transfer组件: 左侧可选数据库→右侧已授权数据库, 搜索/勾选/箭头移动
- role_permissions.py更新: 2值权限映射, get_accessible_instances按access_type过滤
- 后端schema/模型校验更新为readonly|write正则
- 15个测试全部通过

frontend/src/api/instanceGroup.ts

@@ -6,7 +6,7 @@ export interface InstanceGroup {
   description?: string;
   db_type: string;
   permissions?: string[];
-  access_type?: 'readonly' | 'datachange' | 'ddl';
+  access_type?: 'readonly' | 'write';
   allowed_databases?: string[];
   is_active: boolean;
   created_at?: string;