Expire Write Token

Force-expire a write token immediately by setting its expiry to now.

Refused for a token belonging to a frontend application. This route mints nothing to replace the token, and an application's token can only be attached by creating the application, so expiring one leaves the application alive, configured, and unable to receive data until somebody rotates it. Rotate it instead, with grace_period_hours if the bundle carrying it has to be rebuilt first, or delete the application to retire the credential.