GET /api/billing/payments/{id} — 获取支付状态

curl -H "Authorization: Bearer eyJhbGciOi..." \
  https://ariacompute.cn/api/billing/payments/pay_01H...

{
  "id": "pay_01H...",
  "provider": "wechat",
  "amount": 100,
  "currency": "CNY",
  "status": "paid",
  "paid_at": "2026-09-23T10:03:12Z",
  "provider_reference": "wx20260..."
}