Post rendering

[andrewjohnson26]

Title: Post rendering

Description
Transform [media:<url>] tokens into inline embeds:

• .mp4 / .webm → <video controls preload="metadata" src="...">
• .gif → <img src="...">

Estimated Effort: 3h
Dependencies: #443 #444 #446

Acceptance Criteria

• Posts render playable videos inline.
• GIFs render as normal images.
• Works in topic list and full thread view.