Upload a document for a merchant. Requires multipart/form-data with fields: file (binary) and documentType (string)

Example response

{
  "success": true,
  "data": {
    "id": "550e8400-e29b-41d4-a716-446655440000",
    "documentType": "CERTIFICATE_OF_INCORPORATION",
    "fileName": "certificate.pdf",
    "uploadedAt": "2024-01-15T10:30:00.000Z"
  }
}