Firebase Authentication driver for togo auth.
The frontend signs in with the Firebase SDK and POSTs the ID token to the backend,
which verifies it (RS256 against Google certs) and issues a togo session.
Package firebase verifies Firebase Authentication ID tokens and issues a togo
session. The frontend signs in with the Firebase SDK and POSTs the ID token to
/api/auth/firebase; this driver verifies it (RS256 against Google's public
certs, with issuer/audience checks) and find-or-creates the user via auth.