Record Signup

Count the signed-in account as a signup through ?ref=CODE.

Called once by the frontend on a new account's first session, when its browser still holds a referral code (lib/affiliate-ref.ts). Authenticated, so the account is the caller's own; the service ignores accounts older than a week, the affiliate's own account, and anything already recorded.

Visibility only — this never affects commission (ToS 3.2). Always 200 {"received": true}, for the same reason /affiliate/click is.