Record Registration

Send the Meta CompleteRegistration conversion for the caller's account.

Called by the frontend on a new account's first session (PostHogAuthBridge), next to the pixel's sign_up push. The browser call is the only sign-up moment that has the client's IP, user agent and Meta cookies — the users-INSERT webhook has none of them. event_id is the user id, matching the pixel push, so Meta keeps one of the pair.

Authenticated, so the account is the caller's own; accounts older than REGISTRATION_WINDOW are ignored, so replaying the call later can't count a second registration. Always 200 {"received": true}: the browser only needs to know it can stop reporting, and a conversion is best-effort — a DB or queue hiccup is logged, never surfaced.