media assets: add ability to search by AI tags.

Add ability to search the /media_assets index by AI tags. Multi-tag
searches are supported, including AND/OR/NOT operators, but metatags
aren't supported. Multi-tag searches will probably be slow.

The default AI tag confidence threshold is 50%. There's a hidden
search[min_score] URL param that lets you change this.

app/logical/ai_tag_query.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+# An AITagQuery is a tag search performed on media assets using AI tags. Only
+# basic tags are allowed, no metatags.
+class AITagQuery
+  extend Memoist
+
+  attr_reader :search_string
+  delegate :to_infix, :to_pretty_string, to: :ast
+  alias_method :to_s, :to_infix
+
+  def initialize(search_string)
+    @search_string = search_string.to_s.strip
+  end
+
+  def self.search(search_string, **options)
+    new(search_string).search(**options)
+  end
+
+  def ast
+    @ast ||= PostQuery::Parser.parse(search_string)
+  end
+
+  def normalized_ast
+    @normalized_ast ||= ast.to_cnf.rewrite_opts.trim
+  end
+
+  def search(relation: MediaAsset.all, foreign_key: :id, score_range: (50..))
+    normalized_ast.visit do |node, *children|
+      case node.type
+      in :all
+        relation.all
+      in :none
+        relation.none
+      in :tag
+        ai_tag = AITag.named(node.name).where(score: score_range)
+        relation.where(ai_tag.where(AITag.arel_table[:media_asset_id].eq(relation.arel_table[foreign_key])).arel.exists)
+      in :metatag
+        relation.none
+      in :wildcard
+        relation.none
+      in :not
+        children.first.negate_relation
+      in :and
+        joins = children.flat_map(&:joins_values)
+        orders = children.flat_map(&:order_values)
+        nodes = children.map { |child| child.joins(joins).order(orders) }
+        nodes.reduce(&:and)
+      in :or
+        joins = children.flat_map(&:joins_values)
+        orders = children.flat_map(&:order_values)
+        nodes = children.map { |child| child.joins(joins).order(orders) }
+        nodes.reduce(&:or)
+      end
+    end
+  end
+end