feat(push): add telegram bot push notification channel

Implement TelegramPusher in pkg/push, register config schema in channels_definition.go, add validation in model/push_channel.go, update task routing, and update settings-tab.tsx UI validation.

pkg/push/push.go

@@ -10,7 +10,10 @@ import (
 	"sync"
 )
 
-const defaultTitle = "系统通知"
+const (
+	defaultTitle = "系统通知"
+	levelInfo    = "INFO"
+)
 
 // Config 基础通知渠道配置
 type Config struct {