Fix: Add missing migration file

Forgot to include this file in the commit modifying the DB schema.

db/migrate/20250113131902_add_index_to_alert_runs_run_on.rb

@@ -0,0 +1,7 @@
+class AddIndexToAlertRunsRunOn < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index :alert_runs, :run_on, algorithm: :concurrently
+  end
+end