refactor(msg_gateway): decouple bot gateway and push notification architecture

- Split shared monolithic consts into bot, push, and errs with typed sentinel errors
- Restructure model layer into distinct bot and push subdomains
- Refactor DAO layer to enforce single-owner principle and remove cross-table raw SQL queries
- Decompose 1150+ line service/push.go into push_channel, push_event, push_trigger, push_worker, and push_template
- Clean up controller layer with generic request handlers and parameter validation in controller/base.go
- Streamline plugin.go to core Cordis lifecycle orchestration and remove re-export bloat
- Verify all unit tests, race tests, Cordis architecture rules, and Swagger generation pass cleanly

backend/plugins/domain/msg_gateway/consts/push.go

@@ -0,0 +1,43 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package consts
+
+// Push notification channel type constants.
+const (
+	TypeCustom      = "custom"
+	TypeEmail       = "email"
+	TypeTelegram    = "telegram"
+	ChannelCustom   = "custom"
+	ChannelEmail    = "email"
+	ChannelLark     = "lark"
+	ChannelDingTalk = "dingtalk"
+	ChannelTelegram = "telegram"
+	ChannelBark     = "bark"
+	ChannelDiscord  = "discord"
+	ChannelSlack    = "slack"
+	ChannelPushover = "pushover"
+)
+
+// Push message template and payload keys.
+const (
+	DefaultLevelInfo = "INFO"
+	KeyTitle         = "title"
+	KeyContent       = "content"
+	KeyLevel         = "level"
+
+	KeyURL   = "url"
+	KeyToken = "token"
+	KeyOther = "other"
+
+	TypeText     = "text"
+	TypePassword = "password"
+	TypeTextarea = "textarea"
+)
+
+// Push task identifier constants.
+const (
+	TaskPushNotification     = "msg_gateway:push_notification"
+	SendNotificationTask     = "push:send"
+	TaskTypeSendNotification = "send_notification"
+)