Aggiunge newsletter custom con D1, double opt-in e invio campagne.

Attiva il form footer, API subscribe/confirm/unsubscribe e endpoint admin protetti, riusando Resend per le email.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/SiteFooter.astro

@@ -166,6 +166,8 @@ const withLocale = (href: string) => {
                 {notifySubject ? (
                   <input type="hidden" name="_subject" value={notifySubject} />
                 ) : null}
+                <input type="hidden" name="locale" value={currentLocale} />
+                <input type="hidden" name="source" value="footer" />
                 <input
                   type="text"
                   name="_gotcha"