feat(helcim): add transaction list + card update helpers

- listHelcimCustomerTransactions(customerCode): GET /card-transactions/
  with customerCode filter, sorts newest-first, caps at 50, normalizes
  Helcim status (APPROVED/DECLINED) + type (refund) into
  paid/refunded/failed/other.
- updateHelcimCustomerDefaultPaymentMethod(customerId, cardToken):
  resolves cardToken -> cardId via /customers/{id}/cards, then PATCHes
  /customers/{id}/cards/{cardId}/default.
- updateHelcimSubscriptionPaymentMethod(subscriptionId, cardToken):
  wraps updateHelcimSubscription with a cardToken payload.
- helcimUpdateCardSchema: Zod schema { cardToken: string } for the
  upcoming /api/helcim/update-card route.
- Unit tests for all three helpers (success + error paths).

server/utils/schemas.js

@@ -79,6 +79,10 @@ export const helcimSubscriptionSchema = z.object({
   cadence: z.enum(['monthly', 'annual']).default('monthly')
 })
 
+export const helcimUpdateCardSchema = z.object({
+  cardToken: z.string().min(1).max(500)
+})
+
 export const helcimUpdateBillingSchema = z.object({
   customerId: z.union([z.string().min(1), z.number()]),
   billingAddress: z.object({