Fingerprint for second authentication factor, not just convenience

You seem to only use the fingerprint in lieu of a passphrase. I want the ability to force both to be entered as a form of two-factor authentication (something you _know_ **and** something you _have._ And if you ever implement the Yubikey challenge-response protocol, I'd want that to be an optional _third_ factor as well.

No files or tests are named. Start by reviewing the existing fingerprint and passphrase authentication flow, then determine how requiring both factors should be configured and enforced. Done means the requested two-factor behavior is clearly defined and implemented, with YubiKey support treated as a separate optional factor.

Written by the indexing model from the issue text.