Add GET /api/helcim/payment-history returning the authenticated
member's normalized Helcim card transactions (newest first, capped
at 50). Resolves helcimCustomerId -> customerCode via getHelcimCustomer
before calling listHelcimCustomerTransactions. Returns
{ transactions: [] } when the member has no helcimCustomerId, and
{ transactions: [], error: 'unavailable' } (HTTP 200) on Helcim
failures so the UI can render fallback copy.
Covered by unit tests at tests/server/api/helcim-payment-history.test.js
(auth, missing customer id, happy path, both Helcim failure paths,
missing customerCode).
