feat(dkim): make key size configurable and allow keys to be rotated

db/migrate/20260903100000_add_pending_dkim_fields_to_domains.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class AddPendingDKIMFieldsToDomains < ActiveRecord::Migration[7.0]
+
+  def change
+    add_column :domains, :pending_dkim_private_key, :text
+    add_column :domains, :pending_dkim_identifier_string, :string
+  end
+
+end