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/preview.css

@@ -23,3 +23,5 @@
 .editor-add-content{min-height:0;margin-top:12px;padding:10px;border-width:1px;background:#f3f8f4}
 .block-accordion.editor-container>.editor-add-content{margin-top:16px}
 @media(max-width:767px){.editor-empty{padding:16px 10px}.editor-empty button{min-height:44px}}
+/* Die Vorschau läuft ohne Skript; dann blendet Chrome bei jedem Video die Steuerung ein. */
+.section-video::-webkit-media-controls{display:none!important}