How are devices trusted, and what happens on revocation?

At enrollment, each phone generates a key inside its secure hardware; the server keeps only the public half. Every request from the phone is signed — method, path, body hash, timestamp — and no signature is accepted twice. Revoking a device is one action with immediate effect: its keys and refresh-token families are severed, and the user re-establishes access by presenting their face on a new device.