fix: S1 handler display_name + S2 import 移顶部 + W1 注释

- S1: vp_name 硬编码字典 → handler.display_name 属性
- S2: ticker/spawner 中 TaskTypeRegistry 局部 import → 移文件顶部
- W1: TaskHandler executor verify 失败不调 on_failure 加注释说明

src/daemon/mail_handler.py

@@ -20,6 +20,7 @@ class MailHandler(BaseTaskHandler):
 
     task_type = "mail"
     virtual_project = "_mail"
+    display_name = "飞鸽传书"
 
     def target_success_status(self) -> str:
         return "done"