feat(settings): 后台可开关全站评论

关闭后禁止新评论与回复，已有内容仍可浏览；设置经 WS 同步到前台。

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

frontend/lib/realtime.ts

@@ -44,6 +44,7 @@ export interface RtSettingsData {
   site_name?: string;
   site_description?: string;
   allow_register?: boolean;
+  allow_comments?: boolean;
   post_cooldown_hours?: number;
   code_block_auto_fold?: boolean;
   code_block_fold_lines?: number;