[Gekidou] Handle post related websocket events (#5851)

* Handle post related websocket events

* Address feedback and several fixes

* Fix lint

* Address feedback

* LastPostAt as latest post create_at, and lastViewedAt as date.now()

* Address feedback

* Change database for operator and add type imports

types/api/posts.d.ts

@@ -64,7 +64,6 @@ type Post = {
     failed?: boolean;
     user_activity_posts?: Post[];
     state?: 'DELETED';
-    ownPost?: boolean;
     prev_post_id?: string;
     participants: null|string[];
 };