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.

app/pages/member/payment-setup.vue

@@ -72,6 +72,7 @@ const errorMessage = ref('');
 const isProcessing = ref(false);
 const customerId = ref('');
 const customerCode = ref('');
+const hasExistingCard = ref(false);
 
 const initialize = async () => {
   errorMessage.value = '';
@@ -84,13 +85,22 @@ const initialize = async () => {
   }
 
   try {
-    const customer = await $fetch('/api/helcim/get-or-create-customer', {
-      method: 'POST',
-    });
+    // Skip HelcimPay verify if a card's already on file — Helcim refuses
+    // to re-save it, breaking retries after a partial-failed signup.
+    const [customer, existing] = await Promise.all([
+      $fetch('/api/helcim/get-or-create-customer', { method: 'POST' }),
+      $fetch('/api/helcim/existing-card').catch((err) => {
+        console.warn('[payment-setup] existing-card lookup failed, falling back to verify flow:', err);
+        return null;
+      }),
+    ]);
     customerId.value = customer.customerId;
     customerCode.value = customer.customerCode;
+    hasExistingCard.value = Boolean(existing?.cardToken);
 
-    await initializeHelcimPay(customerId.value, customerCode.value, 0);
+    if (!hasExistingCard.value) {
+      await initializeHelcimPay(customerId.value, customerCode.value, 0);
+    }
     step.value = 'ready';
   } catch (err) {
     console.error('Payment setup init failed:', err);
@@ -106,16 +116,18 @@ const openModal = async () => {
   errorMessage.value = '';
 
   try {
-    const result = await verifyPayment();
-    if (!result?.success) throw new Error('Payment was not completed.');
+    if (!hasExistingCard.value) {
+      const result = await verifyPayment();
+      if (!result?.success) throw new Error('Payment was not completed.');
 
-    await $fetch('/api/helcim/verify-payment', {
-      method: 'POST',
-      body: {
-        cardToken: result.cardToken,
-        customerId: customerId.value,
-      },
-    });
+      await $fetch('/api/helcim/verify-payment', {
+        method: 'POST',
+        body: {
+          cardToken: result.cardToken,
+          customerId: customerId.value,
+        },
+      });
+    }
 
     // Update circle first if it changed — update-contribution only touches tier.
     if (targetCircle.value && targetCircle.value !== memberData.value?.circle) {