We want an existing card to be automatically marked as the default during a new subscription purchase

🔍 Problem Statement

When a customer adds a new payment method during a subscription flow (or via the Default UI), it is not set as the default. The old card remains the default — which can cause failed renewals if it is expired or revoked — and customers must manually change the default before they can remove the old card, adding unnecessary friction.

💡 User Story

As a subscriber, I want a newly added payment method to become the default — automatically and/or via an explicit "Set as default" option in the add-payment flow — so that future renewals use my latest card without a separate manual step.

🎯 Definition of Done (DoD)

• ✔ Given a customer who adds a new card during a subscription flow, when the card is saved, then it can be marked as the default payment method for the account (automatically, and/or via a "Set as default payment method" checkbox in the Default UI add-payment modal).
• ✔ Behaviour is consistent across the subscription update flow and the Default UI payment-method modal.
• ✔ This change will impact UI / API — the Default UI payment modal and the payment-method resource (Core API).