add index on submission events

db/migrate/20240228074821_add_index_on_submission_events.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddIndexOnSubmissionEvents < ActiveRecord::Migration[7.1]
+  def change
+    add_index :submission_events, :created_at
+  end
+end