A small open-source service that holds the APNs key and FCM credentials and
sends background push to paired phones on the desktop's behalf. Hosts
authenticate with a box challenge and HMAC proof on their pairing key, the
same shape the relay uses, so signed-in and accountless desktops share one
path. Tokens are stored; alert text is held only for the coalescing window.
The contract doc in docs/reference is the source of truth for every wire
shape. The interop test runs the real desktop answerer against a real
gateway-issued challenge so transcript drift fails in CI.
Jinwoo-H