Add support of video attachment

Implement the ability for users to attach a video when creating a post. The system should allow uploading a video file, store it, view the attached video directly in the feed and on the post page.

Acceptance Criteria:

User can upload a video when creating a post. The video is stored and associated with the post. The post displays a playable video. Unsupported formats are rejected with a clear error message. Compression for video transfer???