Add videos: library, block and section background

Videos join the media library as their own table (schema v28), with a
video block and a looping background on sections. video.js starts loops
only while visible, offers a pause button and leaves the still image for
reduced motion and data saving.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PWYm8kjAWmZaB2QB2xuBhU

public/assets/library-previews.js

@@ -58,6 +58,7 @@ export const PREVIEWS = {
   form: '<rect x="2.5" y="2" width="19" height="3.6" rx="1.2"/><rect x="2.5" y="6.8" width="19" height="3.6" rx="1.2"/><rect x="2.5" y="11.6" width="8" height="3.4" rx="1.2" fill="currentColor" stroke="none"/>',
   legal: `${bar(2, 4, 20, 1.6)}${bar(2, 7.4, 16, 1.6)}${bar(2, 10.8, 11, 1.6)}`,
   partialref: '<rect x="2.5" y="4.5" width="13" height="9" rx="1.5"/><path d="M6.5 4.5V2h13v9h-2.5"/>',
+  video: '<rect x="2" y="2.5" width="20" height="11" rx="1.5"/><path d="M10 5.4l5 2.6-5 2.6z"/>',
   embed: '<rect x="2" y="2.5" width="20" height="11" rx="1.5"/><path d="M10.2 5.6l4.6 2.4-4.6 2.4z" fill="currentColor" stroke="none"/>',
 };