curl -X GET "https://api.nevatal.com/api/v1/billing/invoices/inv_001" \
  -H "Authorization: Bearer nvtl_your_api_key"

{
  "id": "inv_001",
  "invoiceNumber": "FV-2025-0042",
  "patientId": "pat_abc123",
  "patient": {
    "firstName": "María",
    "lastName": "García",
    "documentType": "CC",
    "documentNumber": "1234567890"
  },
  "status": "approved",
  "subtotal": 150000,
  "discount": 0,
  "tax": 28500,
  "total": 178500,
  "currency": "COP",
  "items": [
    {
      "description": "Consulta General",
      "procedureId": "proc_general_consult",
      "quantity": 1,
      "unitPrice": 150000,
      "taxRate": 0.19,
      "total": 150000
    }
  ],
  "payments": [],
  "createdAt": "2025-03-15T10:30:00Z",
  "approvedAt": "2025-03-15T11:00:00Z"
}