Get Signup Availability

Whether public self-serve signup is currently open.

Rate Limited: 30 requests per minute per IP address.

Unauthenticated pre-flight for the signup page, so a closed surface shows its closed screen BEFORE the form can mint a Firebase identity the POST would then refuse. Advisory only: the toggle-gated POST routes remain the authoritative gate, and the closed state already leaks through their 404s, so this read reveals nothing new.