Get Saved Card

Describe the payment method on file, so the UI can name it.

Its own endpoint rather than a field on GET /d/credits: that one is hit on every page, filter and pagination change, and none of those should wait on a Paddle round trip. This is read once when the card row renders.

Never fails the caller. Paddle being slow, the customer being unknown, or the method having been removed all come back as method=null, and the UI falls back to "payment method saved" — a named card is a convenience, not something worth breaking a billing page over.