curl --request POST \
  --url https://api.sprucecollective.co.uk/auth/check-or-create-user \
  --header 'Content-Type: application/json' \
  --data '
{
  "email": "<string>",
  "site_id": "<string>"
}
'

{
  "error": "<string>",
  "exists": true,
  "message": "<string>",
  "success": true,
  "user_id": "<string>"
}