refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/message_gateway/custom_events_admin_login.go

@@ -0,0 +1,42 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import (
+	"context"
+	"time"
+
+	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+)
+
+// AdminLogin is the metadata definition for the admin login event.
+var AdminLogin = EventMetadata{
+	Key:  "admin_login",
+	Name: "管理员登录",
+	DefaultTemplate: NotificationMessage{
+		Title:   "管理员登录提醒",
+		Content: "管理员 {{user.username}} 于 {{time}} 从 IP {{ip}} 登录系统。",
+		Level:   "INFO",
+	},
+	Description: "当管理员成功登录系统时触发此通知",
+}
+
+// HandleAdminLoggedIn 处理管理员登录事件并触发通知
+func HandleAdminLoggedIn(ctx context.Context, event contracts.AdminLoggedIn) {
+	if event.User == nil {
+		return
+	}
+
+	body := map[string]any{
+		"user": event.User,
+		"ip":   event.IP,
+		"time": time.Now().Format("2006-01-02 15:04:05"),
+	}
+	DefaultTrigger.Trigger(ctx, AdminLogin, body)
+}
+
+// RegisterCustomEvents registers default domain push notification events.
+func RegisterCustomEvents() {
+	RegisterBuiltInEvent(AdminLogin)
+}