curl https://substrate-engine.fly.dev/v1/respond \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "entity": "my-entity",
    "input": "Hello, I would like to have a conversation."
  }'

const res = await fetch('https://substrate-engine.fly.dev/v1/respond', {
  method: 'POST',
  headers: {
    'Authorization': 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    entity: 'my-entity',
    input: 'Hello!'
  })
});
const data = await res.json();