Helcim refuses paymentType:'verify' for cards already saved on a
customer ("A new card must be entered for saving the payment method"),
breaking every "Complete Payment" retry after a partial-failed signup.
Add GET /api/helcim/existing-card and short-circuit HelcimPay verify in
useMemberPayment + payment-setup.vue when a saved card is found, going
straight to subscription creation. The two existence-check fetches run
in parallel with get-or-create-customer so no extra round-trip latency
in the common path.
