curl --request GET \
  --url https://api.erna.ai/v1/billing/transactions \
  --header 'Authorization: Bearer <token>'

{
  "transactions": [
    {
      "id": "txn_123",
      "userId": "user_123",
      "amount": -125,
      "type": "usage",
      "reference": "run_123",
      "description": "Schedule generation",
      "createdAt": "2026-04-20T12:00:00.000Z"
    }
  ]
}