refactor(plugins): restructure admin and message_gateway into standard layered sub-packages

backend/plugins/domain/message_gateway/model/admin.go

@@ -0,0 +1,46 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package model
+
+// Field is one admin form field.
+type Field struct {
+	Key      string `json:"key"`
+	Type     string `json:"type"`
+	Required bool   `json:"required"`
+}
+
+// Definition describes a channel type form.
+type Definition struct {
+	Type   string  `json:"type"`
+	Fields []Field `json:"fields"`
+}
+
+// ChannelDTO represents a channel for admin consumption.
+type ChannelDTO struct {
+	ID          uint64            `json:"id,string"`
+	Name        string            `json:"name"`
+	Type        string            `json:"type"`
+	OwnerScope  string            `json:"owner_scope"`
+	OwnerID     *uint64           `json:"owner_id,string,omitempty"`
+	Enabled     bool              `json:"enabled"`
+	Credentials map[string]string `json:"credentials"`
+	Extra       map[string]string `json:"extra"`
+}
+
+// CreateChannelRequest is admin create payload.
+type CreateChannelRequest struct {
+	Name        string            `json:"name"`
+	Type        string            `json:"type"`
+	Enabled     *bool             `json:"enabled"`
+	Credentials map[string]string `json:"credentials"`
+	Extra       map[string]string `json:"extra"`
+}
+
+// UpdateChannelRequest is admin update payload.
+type UpdateChannelRequest struct {
+	Name        string            `json:"name"`
+	Enabled     *bool             `json:"enabled"`
+	Credentials map[string]string `json:"credentials"`
+	Extra       map[string]string `json:"extra"`
+}