ai tags: replace tag_id index with (tag_id, score) index.

Index on (tag_id, score) instead of (tag_id) to allow tags to be
filtered and sorted by confidence more efficiently. This index takes up
about the same amount of space as an index on tag_id alone, so including
the score in the index is essentially free.

db/migrate/20220627211714_add_tag_id_and_score_index_to_ai_tags.rb

@@ -0,0 +1,6 @@
+class AddTagIdAndScoreIndexToAITags < ActiveRecord::Migration[7.0]
+  def change
+    add_index :ai_tags, [:tag_id, :score], if_not_exists: true
+    remove_index :ai_tags, :tag_id
+  end
+end