列出交易記錄

{
  "transactions": [
    {
      "id": "txn_xxx",
      "type": "usage",
      "amount": -0.0042,
      "description": "openai/gpt-5.4 - 500 input tokens, 200 output tokens",
      "created_at": "2026-04-03T10:00:00Z"
    },
    {
      "id": "txn_yyy",
      "type": "credit",
      "amount": 50.0,
      "description": "Credit purchase",
      "created_at": "2026-04-01T09:00:00Z"
    }
  ]
}