Implement support for enabling Two-Factor Authentication (2FA) and completing the verification process.

[ewout05]

Support enabling 2FA during account setup.
Handle OTP (One-Time Password) verification.
Provide a way to supply the OTP manually or through an external source (e.g. an API or automation workflow).
Return clear errors if verification fails or the OTP expires.
Design the implementation so automated OTP providers can be integrated later.