Parse a Typeform form_response webhook payload and map answers back to form fields

Documented steps

1. Auth: None — inbound payload; authenticate via Typeform-Signature HMAC verification instead of a bearer token.
2. Top level: `event_id` (unique per delivery), `event_type` (e.g. "form_response"), and a nested `form_response` object.
3. `form_response.token` is the unique submission ID — the same response ID used by the Responses API; use it for dedup/idempotency (NOT event_id, which identifies the delivery attempt).
4. `form_response.definition` (id, title, fields[]) describes the form structure at submission time; `form_response.answers[]` holds submitted values.
5. Each answer has `type` (text, choice, choices, email, date, boolean, number, file_url, payment, url, signature, ...), a `field` object (id, type, ref), and a value keyed BY the answer's type (e.g. `text`, `choice: {label}`, `number`).
6. Map answers to questions by `answer.field.ref` (preferred — you control refs at form-creation time) or `answer.field.id` against `definition.fields[]`.
7. `form_response.hidden` holds hidden-field key/values; `form_response.calculated.score` holds scoring; `form_response.variables[]` holds form variables.
8. Unanswered fields are simply omitted from answers[] — never rely on array position matching definition.fields order.
9. Official docs: https://www.typeform.com/developers/webhooks/example-payload/