Join the waitlist. Public, unauthenticated, and ALWAYS {ok:true} for a well-formed email — a duplicate is a silent no-op (INSERT … ON CONFLICT DO NOTHING), so nobody can probe who is already on the list.

Order (deliberate): per-IP rate limit FIRST (its own limiter, before any DB work — this is an anonymous write route, same posture as /v1/events), then email shape validation (honest 400 — that's form feedback, not enumeration), then the best-effort insert.