ai tags: make ai tags accessible in api via includes.

Make these things work:

* https://danbooru.donmai.us/posts.json?only=ai_tags
* https://danbooru.donmai.us/media_assets.json?only=ai_tags
* https://danbooru.donmai.us/ai_tags.json?only=media_asset,post,tag

app/models/post.rb

@@ -69,6 +69,7 @@ class Post < ApplicationRecord
   has_many :disapprovals, :class_name => "PostDisapproval", :dependent => :destroy
   has_many :favorites, dependent: :destroy
   has_many :replacements, class_name: "PostReplacement", :dependent => :destroy
+  has_many :ai_tags, through: :media_asset
 
   attr_accessor :old_tag_string, :old_parent_id, :old_source, :old_rating, :has_constraints, :disable_versioning, :post_edit
 
@@ -1639,7 +1640,7 @@ class Post < ApplicationRecord
     %i[
       uploader approver flags appeals parent children notes
       comments approvals disapprovals replacements pixiv_ugoira_frame_data
-      artist_commentary
+      artist_commentary media_asset ai_tags
     ]
   end
 end