refactor: Simplify root user notification and remove global email variable

- Remove global `RootUserEmail` variable
- Modify channel testing and user notification methods to use `GetRootUser()`
- Update user cache and notification service to use more consistent user base type
- Add new channel test notification type

dto/notify.go

@@ -12,6 +12,7 @@ const ContentValueParam = "{{value}}"
 const (
 	NotifyTypeQuotaExceed   = "quota_exceed"
 	NotifyTypeChannelUpdate = "channel_update"
+	NotifyTypeChannelTest   = "channel_test"
 )
 
 func NewNotify(t string, title string, content string, values []interface{}) Notify {