Materials: images with captions and video support

- markdown: new directives `:::figure assets/x.png | Подпись` and
  `:::video assets/x.mp4 | Подпись` rendered as figure/figcaption and a
  video player (works with HTML escaping on); sanitizer allows video/source/
  track/figure with safe attributes only
- assets: video extensions (mp4/webm/ogg) allowed, separate limits — images
  up to 10 MB, video up to 100 MB; correct content types
- upload limits raised: web/academic multipart 120 MB per file / 200 MB per
  request, nginx client_max_body_size 200m (covers media inside ZIP imports)
- docs: new docs/06-authoring.md (markdown, images, video, assets, limits,
  import semantics); import message now says "медиафайлов"

web/src/main/resources/application.yml

@@ -16,8 +16,8 @@ spring:
     name: web
   servlet:
     multipart:
-      max-file-size: 30MB
-      max-request-size: 120MB
+      max-file-size: 120MB
+      max-request-size: 200MB
   thymeleaf:
     cache: false