Fix #5294: pixiv_ugoira_frame_data alternative

Allow including the media metadata with the post like this:

* https://danbooru.donmai.us/posts.json?tags=ugoira&only=id,file_url,media_metadata

app/models/post.rb

@@ -1630,7 +1630,7 @@ class Post < ApplicationRecord
     %i[
       uploader approver flags appeals events parent children notes
       comments approvals disapprovals replacements
-      artist_commentary media_asset ai_tags
+      artist_commentary media_asset media_metadata ai_tags
     ]
   end
 end