Request Gift Extension

A visitor's 72h gift-scan countdown lapsed before they claimed it — let them ask for one more 24h window. Capped at one request ever per email (the UNIQUE index below is what enforces that, same pattern as the gift_scans reservation above). Unless they've already used a free scan, this sends a confirmation email with a one-time link — the 24h window only actually starts once that link is clicked, so typing in a bogus or someone-else's address can't grant anything by itself.