golem15.user.device_auth.auth_url gains a {short_code} placeholder and
defaults to {frontend_url}/device/scan/{short_code}; POST
/devices/authorize accepts the short code in its token field, so a scan
page that posts what its URL carries keeps working. The status poll hands
the session token over only to a client with the User-Agent the device
initiated with; another client sees the session confirmed and takes
nothing.
PHP's device flow on golem15_user_device_auth_sessions (PHP's columns,
the token stored as its SHA-256, plus jwt_jti): POST /devices/initiate
answers a 64-character token, an XXXX-XXXX short code, the configurable
auth_url (golem15.user.device_auth.auth_url) and the expiry; a signed-in
user authorizes it with POST /devices/authorize (short_code or token,
one guarded pending-to-confirmed UPDATE); the device's first poll of
GET /devices/status/{token} after that receives a session JWT through one
guarded confirmed-to-used UPDATE, so exactly one poll gets it. Used and
revoked sessions never turn into expired; each initiate deletes a bounded
batch of stale sessions.
