fix(helcim): skip HelcimPay verify when a card is already on file

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.

server/api/helcim/existing-card.get.js

@@ -0,0 +1,25 @@
+// Returns the saved cardToken so callers can skip HelcimPay re-entry.
+import { requireAuth } from '../../utils/auth.js'
+import { listHelcimCustomerCards } from '../../utils/helcim.js'
+
+export default defineEventHandler(async (event) => {
+  const member = await requireAuth(event)
+
+  if (!member.helcimCustomerId) {
+    return { cardToken: null }
+  }
+
+  const cardsResponse = await listHelcimCustomerCards(member.helcimCustomerId)
+  const cards = Array.isArray(cardsResponse)
+    ? cardsResponse
+    : (cardsResponse?.cards || cardsResponse?.data || [])
+
+  if (!cards.length) {
+    return { cardToken: null }
+  }
+
+  const defaultCard =
+    cards.find((c) => c?.default === true || c?.isDefault === true) || cards[0]
+
+  return { cardToken: defaultCard?.cardToken || null }
+})