Passkeys

The signed-in person's passkeys.


`GET passkeys`	a `PasskeyList`
`POST passkeychallenges`	options for `navigator.credentials.create()`: `{ "spec": { "purpose": "Register" } }`
`POST passkeys`	register one: `{ "spec": { "credential": { … }, "displayName": "…" } }`
`PATCH passkeys/<name>`	rename: `{ "spec": { "displayName": "…" } }`
`DELETE passkeys/<name>`	remove

A challenge answers once, within five minutes. A new passkey is named for the device that made it, "Passkey synced from Mac", until it is renamed. Signing in with a passkey happens in an interaction.