:::endpoint POST /v1/me/calendar/events/{id}/respond

An invitation that arrives as mail lands on the calendar by itself, with
`status: "pending"`, and holds its place until somebody answers.

Two answers and no third. Accepting confirms it and writes your line on the
guest list. Ignoring takes it off the calendar outright, which is what ignore
means: the mail is still in the mailbox with the invitation attached, so nothing
is lost and it can be accepted from there again.

:::note Declining is not here
Telling an organiser no is sending them a reply, which is a different thing from
taking their invitation off your own calendar. Only the second is in this API.
:::

## Request

| Field    | Required | Description          |
| -------- | -------- | -------------------- |
| `answer` | yes      | `accept` or `ignore` |

```bash
curl -X POST https://api.proppertrading.com/v1/me/calendar/events/4490/respond \
  -H "Authorization: Bearer ptat_9f2c1a44_3b8e7d2f5c9a1b4e6d8f0a2c4e6b8d1f" \
  -H "Idempotency-Key: 8f14e45f-ea0d-4d1f-9c4b-3e11f0f2c0a1" \
  -H "Content-Type: application/json" \
  -d '{ "answer": "accept" }'
```

## Response

`200` with the confirmed appointment on `accept`, and `204` with no body on
`ignore`.

## Errors

| Status | Code                      | Meaning                                                           |
| ------ | ------------------------- | ----------------------------------------------------------------- |
| 400    | `missing_idempotency_key` | The `Idempotency-Key` header was not sent. Every `POST` needs one |
| 400    | `invalid_answer`          | The answer was not `accept` or `ignore`                           |
| 400    | `not_an_invitation`       | That appointment is not waiting for an answer                     |
| 403    | `insufficient_scope`      | This device does not hold `calendar:write`                        |
| 403    | `calendar_read_only`      | The account may read that calendar but not write in it            |
| 404    | `resource_not_found`      | No such appointment here                                          |