Add “Mark as Refunded” status

Add a refund action on invoices/payments so an admin can mark a transaction as Refunded inside ManyRequests. Once marked, the refund should:

• Show a clear Refunded label/status in the relevant views (invoice, client billing history, transactions list).

• Be reflected in reporting by deducting the refunded amount from Revenue (net revenue), so revenue metrics and dashboards stay accurate.