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/translations.ts

@@ -353,6 +353,22 @@ const TRANSLATION_KEYS = [
   "mitmachen_error_files_count",          // {{n}}
   "mitmachen_error_files_size",           // {{mb}}
   "mitmachen_error_files_type",
+  // Newsletter form (NewsletterFormComponent.svelte). Generische Submit-/
+  // Server-Error-Texte werden aus contact_* wiederverwendet.
+  "newsletter_label_name",
+  "newsletter_placeholder_name",
+  "newsletter_label_email",
+  "newsletter_placeholder_email",
+  "newsletter_label_consent",
+  "newsletter_privacy_note",
+  "newsletter_submit",
+  "newsletter_submitting",
+  "newsletter_success_title",
+  "newsletter_success_body",
+  "newsletter_error_email_required",
+  "newsletter_error_email_invalid",
+  "newsletter_error_email_max",           // {{n}}
+  "newsletter_error_consent_required",
 ] as const;
 
 export type TranslationKey = (typeof TRANSLATION_KEYS)[number];