Single opt-in newsletter subscribe (form-encoded)

Single opt-in email capture for the directory digest. Content-Type MUST be application/x-www-form-urlencoded (or multipart/form-data); because it is a form POST, Astro's cross-origin protection requires a same-origin Origin header. Field: email (required); optional source, interest. Responds with a self-contained text/html confirmation page (200). The intake is rate-limited per IP per hour.