fixup! Add index to submissions

Author: SamJamCul

db/migrate/20260218110428_add_index_on_created_at_and_form_id_and_mode_to_submissions.rb

@@ -0,0 +1,5 @@
+class AddIndexOnCreatedAtAndFormIdAndModeToSubmissions < ActiveRecord::Migration[8.1]
+  def change
+    add_index :submissions, %i[created_at form_id mode]
+  end
+end