curl --request GET \
  --url https://api.sprucecollective.co.uk/reports/returning-customers \
  --header 'Authorization: <api-key>'

{
  "customers": [
    {
      "average_order_value": 123,
      "customer_lifetime_value": 123,
      "days_between_purchases": 123,
      "email": "<string>",
      "first_name": "<string>",
      "first_purchase_date": "<string>",
      "is_active": true,
      "last_name": "<string>",
      "last_purchase_date": "<string>",
      "purchase_frequency": 123,
      "total_spent_cents": 123,
      "total_spent_dollars": 123,
      "total_transactions": 123,
      "user_id": "<string>"
    }
  ],
  "summary": {
    "active_returning_customers": 123,
    "average_customer_lifetime_value": 123,
    "average_days_between_purchases": 123,
    "average_purchase_frequency": 123,
    "top_spender": {
      "email": "<string>",
      "total": 123
    },
    "total_returning_customers": 123,
    "total_revenue_from_returning": 123
  }
}