Get spend summary

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

{
  "ok": true,
  "data": {
    "period_days": 30,
    "total_purchased": "150.000000",
    "total_consumed": "54.770000",
    "transaction_count": 23,
    "currency": "USD"
  },
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}