Test mode: no way to simulate a failed subscription renewal (dunning untestable) #162

The documented failure test cards (4507 9900 0000 0028 / 0010 / 0044) are rejected by save-time card verification everywhere they could attach to a subscription. The customer portal's "Update Payment Details" and even a $0-due trial checkout both decline them. That makes them checkout-only simulators, and leaves no way to take a healthy test sub into past_due/dunning to verify what customers experience (emails, retry cadence, webhook sequence) before going live. Request: either a test card that succeeds on first auth and fails on subsequent charges (à la Stripe's 4000 0000 0000 0341), or a dashboard/API "simulate renewal failure" action in test mode.