{
  "ok": true,
  "service": "chgwatch",
  "price_usd": "0.01",
  "free": [
    "/watch?url=...&mode=text|markdown|meta|links[&store=true]"
  ],
  "paid": [
    "/diff?url=...&prev_hash=... | &prev_text=... | &baseline=stored"
  ],
  "flow": "1) GET /watch and save hash. 2) later GET /diff with prev_hash=that hash. changed=true|false plus added/removed lines."
}