GET /api/billing/usage — 计量用量记录

curl -H "Authorization: Bearer eyJhbGciOi..." \
  "https://ariacompute.cn/api/billing/usage?from=2026-09-01T00:00:00Z&granularity=day"

{
  "usage": [
    {
      "timestamp": "2026-09-22T00:00:00Z",
      "model": "gemma-4-e2b-it_q4",
      "quantity": 1250000,
      "unit": "per_million_tokens",
      "cost": 1.50,
      "currency": "CNY"
    }
  ]
}