feat: 后台增加开放消息开关

关闭后前台隐藏私聊/群聊入口并由后端拦截；同步去掉公告页重复计数胶囊。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/realtime.ts

@@ -45,6 +45,7 @@ export interface RtSettingsData {
   site_description?: string;
   allow_register?: boolean;
   allow_comments?: boolean;
+  allow_messages?: boolean;
   post_cooldown_hours?: number;
   code_block_auto_fold?: boolean;
   code_block_fold_lines?: number;