feat(newsletter): newsletter signup as internal component

Wire a newsletter signup form into the CMS internal-component pattern
(form-newsletter). Posts to /api/newsletter which subscribes to listmonk
(double opt-in) and mirrors a best-effort backup record to the RustyCMS
forms plugin. Reuses the contact-form anti-spam stack: Cloudflare
Turnstile, honeypots, client-age check, origin validation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/block-types.ts

@@ -469,7 +469,7 @@ export type CalendarBlockData = Omit<
 };
 
 /** Internal component block (_type: "internal_component"). */
-export type InternalComponentId = "form-contact" | "form-mitmachen";
+export type InternalComponentId = "form-contact" | "form-mitmachen" | "form-newsletter";
 
 export interface InternalComponentBlockData {
   _type?: "internal_component";