Preview a product invite (sign-up vs sign-in routing).

MNE-8335. Unauthenticated and keyed by the secret pinv_… token — the same secret that authorises accept. Lets the website send a logged-out invitee to /signup (no account yet) or /login (already has one) before they authenticate. Returns only the effective status, the invited products, and needs_account — true only for a LIVE email-bound user invite whose address has no account yet (decided server-side from the invite record; any failed lookup yields false, i.e. sign-in). Never returns an email, an id or a grant amount, and there is deliberately no email-keyed variant (MNE-1348 anti-enumeration). A live row past its expiry reports expired. Unknown token: 404.