List customers

curl https://api.example.com/v1/customers?limit=2 \
  -H "Authorization: Bearer sk_test_..."

{
  "data": [
    { "id": "cus_001", "name": "Ada Lovelace", "email": "ada@example.com" },
    { "id": "cus_002", "name": "Alan Turing", "email": "alan@example.com" }
  ],
  "has_more": false
}