POST /api/helcim/update-card updates the customer's default card, then
best-effort patches the active subscription payment method. Status-gated
to {active, pending_payment}; verifies the submitted cardToken is
attached to the member's helcimCustomerId via listHelcimCustomerCards.
On subscription PATCH 5xx we revert the customer default to the prior
card token; 4xx (schema rejection — cardToken is not a documented
subscription PATCH field) is tolerated since the customer default is
the authoritative billing driver.
