Toggle Stripe Payment Methods Per Invoice or Customer

Let users control which Stripe payment methods (credit card, ACH, etc.) are available on a per-invoice or per-customer basis, rather than having all Stripe methods enabled across all invoices.

Common use case: high-value invoices where users want ACH-only to avoid card processing fees, while still allowing credit card on smaller recurring invoices. Today there's no way to toggle this without going into Stripe settings directly.