feat(message_gateway): expose PushRegistry contract

backend/core/contracts/push.go

@@ -0,0 +1,26 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package contracts defines unified service interfaces and DTOs for cross-plugin communication.
+package contracts
+
+import "context"
+
+type PushNotificationTemplate struct {
+	Title   string
+	Content string
+	Level   string
+	Ext     map[string]any
+}
+
+type PushEventMeta struct {
+	Key             string
+	Name            string
+	Description     string
+	DefaultTemplate PushNotificationTemplate
+}
+
+type PushRegistry interface {
+	RegisterBuiltInEvent(meta PushEventMeta)
+	SyncEvents(ctx context.Context) error
+}