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

{
  "ok": true,
  "data": {
    "balance": "45.230000",
    "balance_purchased": "100.000000",
    "balance_consumed": "54.770000",
    "currency": "USD",
    "auto_topup": {
      "enabled": true,
      "threshold": "10.000000",
      "amount": "50.000000",
      "monthly_limit": "200.000000",
      "monthly_used": "50.000000"
    }
  },
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}