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

templates/blocks/video-toggle.html.twig

@@ -0,0 +1,2 @@
+{# Sichtbar erst durch video.js - ohne Skript läuft auch nichts, das anzuhalten wäre. #}
+<button class="video-toggle" type="button" hidden aria-label="{{ 'Video anhalten'|kant_t }}" data-pause="{{ 'Video anhalten'|kant_t }}" data-play="{{ 'Video abspielen'|kant_t }}"></button>