test(server): add node --test harness with failing free-tier billing tests

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/lib/billing.js

@@ -828,4 +828,10 @@ module.exports = {
   syncRevenueCatCustomerInfo,
   syncRevenueCatWebhookEvent,
   storeEndpointResponse,
+  // exported for tests
+  buildDefaultAccount,
+  alignAccountToCurrentCycle,
+  getAvailableCredits,
+  consumeCredits,
+  buildBillingSummary,
 };