Add system announcements interface

app/models/system/Announcement.model.js

@@ -0,0 +1,26 @@
+const { Model } = require('flitter-orm')
+
+class AnnouncementModel extends Model {
+    static get schema() {
+        return {
+            title: String,
+            message: String,
+            user_ids: [String],
+            group_ids: [String],
+            type: String, // login | email | banner
+        }
+    }
+
+    async to_api() {
+        return {
+            id: this.id,
+            title: this.title,
+            message: this.message,
+            user_ids: this.user_ids,
+            group_ids: this.group_ids,
+            type: this.type,
+        }
+    }
+}
+
+module.exports = exports = AnnouncementModel