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/site.css

@@ -241,6 +241,16 @@ body.consent-open{padding-bottom:320px}
 .embed-provider{margin:0 0 16px;font-size:13px;color:var(--muted)}
 /* Eigenes HTML: nur so viel Regel, dass fremdes Markup die Spalte nicht sprengt. Alles
    Weitere bestimmt der Code selbst - das ist der Zweck des Bausteins. */
+.block-video{position:relative;margin:0;max-width:100%}
+.video-media{display:block;width:100%;height:auto;border-radius:10px}
+.has-video{position:relative;overflow:hidden}
+.section-video{position:absolute;inset:0;width:100%;height:100%;object-fit:cover;pointer-events:none}
+.has-video>.section-inner{position:relative;z-index:1}
+.video-toggle{position:absolute;right:16px;bottom:16px;z-index:2;width:44px;height:44px;padding:0;border:0;border-radius:50%;background:rgba(10,20,35,.72);color:#fff;font:14px/1 system-ui,sans-serif;cursor:pointer}
+.video-toggle::before{content:"❚❚"}
+.video-toggle.is-paused::before{content:"▶"}
+.video-toggle:focus-visible{outline:3px solid #fff;outline-offset:2px}
+.video-toggle[hidden]{display:none}
 .block-html{max-width:100%}
 .block-html img,.block-html video,.block-html table{max-width:100%}
 .block-html table{border-collapse:collapse}