seo: add json-ld structured data for videos.

Makes video posts eligible for rich results in Google, which means they
can include a thumbnail.

Ref: https://developers.google.com/search/docs/data-types/video

app/helpers/seo_helper.rb

@@ -12,6 +12,19 @@ module SeoHelper
     "#{Danbooru.config.canonical_app_name} is the original anime image booru. Search millions of anime pictures categorized by thousands of tags."
   end
 
+  # https://developers.google.com/search/docs/data-types/video#video-object
+  def json_ld_video_data(post)
+    json_ld_tag({
+      "@context": "https://schema.org",
+      "@type": "VideoObject",
+      "name": page_title,
+      "description": meta_description,
+      "uploadDate": post.created_at.iso8601,
+      "thumbnailUrl": post.preview_file_url,
+      "contentUrl": post.file_url,
+    })
+  end
+
   def json_ld_website_data
     urls = [
       Danbooru.config.twitter_url,