GET /api/billing/summary — wallet and recent activity

curl -H "Authorization: Bearer eyJhbGciOi..." \
  https://ariacompute.com/api/billing/summary

{
  "wallet": { "balance": 42.10, "currency": "USD" },
  "recent_payments": [
    { "id": "pay_01H...", "amount": 20, "currency": "USD", "status": "paid" }
  ],
  "recent_transactions": [
    { "id": "txn_01H...", "type": "usage", "amount": -0.12, "currency": "USD" }
  ],
  "month_usage": { "total_cost": 7.90, "currency": "USD" }
}