Set member status to sustainer upon payment

Introduces a state machine for the payment status as well.

refs #213

db/migrate/20250527113805_update_payment_status_to_pending.rb

@@ -0,0 +1,6 @@
+class UpdatePaymentStatusToPending < ActiveRecord::Migration[8.0]
+  def change
+    Donation.where(payment_status: nil).update_all(payment_status: "pending")
+    Donation.where.not(payment_status: %w[pending processing settled]).update_all(payment_status: "pending")
+  end
+end