Record an opt-in to be told when rates move enough to change a saved analysis.

This is the only durable contact an anonymous visitor ever leaves, and before 2026-09-15 there was no way to leave one at all — a visitor ran one analysis and was gone, which at single-digit weekly traffic is indistinguishable from nobody visiting.

Deliberately NOT an instant-send: nothing here promises mail arrives now, because there is no mail provider wired up and the UI must not claim otherwise. At current volume these are reviewed and sent by hand; automate it when the volume makes that annoying, not before.