Newsletter Subscribe

Capture a newsletter sign-up and email a double opt-in confirmation link.

Creates a new newsletter_subscribers row, or refreshes an existing one for the same email (spec section 5 step 1: "creates (or reuses, if the email already has a row)") -- an already-active subscriber resubmitting the form keeps their active status (a duplicate confirmation email is harmless; losing an existing subscription to an accidental double-submit would not be), every other status is reset to pending_verification with a fresh token.

Same failure-handling shape as submit_feedback(): a failed SEND does not fail the request -- the row is already durably stored -- but is surfaced via email_sent: false rather than hidden.