Add AI tag model and UI.

Add a database model for storing AI-predicted tags, and add a UI for browsing and searching these tags.

AI tags are generated by the Danbooru Autotagger (https://github.com/danbooru/autotagger). See that
repo for details about the model.

The database schema is `ai_tags (media_asset_id integer, tag_id integer, score smallint)`. This is
designed to be as space-efficient as possible, since in production we have over 300 million
AI-generated tags (6 million images and 50 tags per post). This amounts to over 10GB in size, plus
indexes.

You can search for AI tags using e.g. `ai:scenery`. You can do `ai:scenery -scenery` to find posts
where the scenery tag is potentially missing, or `scenery -ai:scenery` to find posts that are
potentially mistagged (or more likely where the AI missed the tag).

You can browse AI tags at https://danbooru.donmai.us/ai_tags. On this page you can filter by
confidence level. You can also search unposted media assets by AI tag.

To generate tags, use the `autotag` script from the Autotagger repo, something like this:

  docker run --rm -v ~/danbooru/public/data/360x360:/images ghcr.io/danbooru/autotagger ./autotag -c -f /images | gzip > tags.csv.gz

To import tags, use the fix script in script/fixes/. Expect a Danbooru-size dataset to take
hours to days to generate tags, then 20-30 minutes to import. Currently this all has to be done by hand.

app/logical/autocomplete_service.rb

@@ -211,6 +211,8 @@ class AutocompleteService
       autocomplete_favorite_group(value)
     when :search
       autocomplete_saved_search_label(value)
+    when :ai, :unaliased
+      autocomplete_tag(value)
     when *STATIC_METATAGS.keys
       autocomplete_static_metatag(metatag, value)
     else

app/logical/concerns/searchable.rb

@@ -430,7 +430,13 @@ module Searchable
     end
 
     if model == Post && params["#{attr}_tags_match"].present?
-      relation = relation.where(attr => Post.user_tag_match(params["#{attr}_tags_match"], current_user).reorder(nil))
+      posts = Post.user_tag_match(params["#{attr}_tags_match"], current_user).reorder(nil)
+
+      if association.through_reflection?
+        relation = relation.includes(association.through_reflection.name).where(association.through_reflection.name => { attr => posts })
+      else
+        relation = relation.where(attr => posts)
+      end
     end
 
     if params["has_#{attr}"].to_s.truthy? || params["has_#{attr}"].to_s.falsy?

app/logical/post_query_builder.rb

@@ -38,7 +38,7 @@ class PostQueryBuilder
     ordpool note comment commentary id rating source status filetype
     disapproved parent child search embedded md5 width height mpixels ratio
     score upvotes downvotes favcount filesize date age order limit tagcount pixiv_id pixiv
-    unaliased exif duration random is has
+    unaliased exif duration random is has ai
   ] + COUNT_METATAGS + COUNT_METATAG_SYNONYMS + CATEGORY_COUNT_METATAGS
 
   ORDER_METATAGS = %w[
@@ -163,6 +163,8 @@ class PostQueryBuilder
       relation.tags_include(value)
     when "exif"
       relation.exif_matches(value)
+    when "ai"
+      relation.ai_tags_include(value)
     when "user"
       relation.uploader_matches(value)
     when "approver"