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"

deploy/nginx/lms.our-place.ru.conf

@@ -23,7 +23,7 @@ server {
     }
 
     # Большие загрузки: ZIP лаб, решения, материалы.
-    client_max_body_size 120m;
+    client_max_body_size 200m;   # крупные ZIP с изображениями и видео
 
     location / {
         proxy_pass http://127.0.0.1:8090;
@@ -52,7 +52,7 @@ server {
 #     ssl_certificate_key /etc/letsencrypt/live/lms.our-place.ru/privkey.pem;
 #     ssl_protocols TLSv1.2 TLSv1.3;
 #
-#     client_max_body_size 120m;
+#     client_max_body_size 200m;   # крупные ZIP с изображениями и видео
 #
 #     location / {
 #         proxy_pass http://127.0.0.1:8090;