add submission created_at index

db/migrate/20260707055354_add_submission_created_at_index.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddSubmissionCreatedAtIndex < ActiveRecord::Migration[8.1]
+  def change
+    add_index :submissions, :created_at, if_not_exists: true
+  end
+end