GET /api/billing/transactions — 钱包交易历史

curl -H "Authorization: Bearer eyJhbGciOi..." \
  "https://ariacompute.cn/api/billing/transactions?limit=20"

{
  "transactions": [
    {
      "id": "txn_01H...",
      "type": "usage",
      "amount": -0.12,
      "currency": "CNY",
      "created_at": "2026-09-23T08:59:00Z",
      "description": "gemma-4-e2b-it_q4 inference"
    }
  ],
  "next_cursor": null
}