feat: Mitmachen-Formular nativ (löst OpnForm ab)

- MitmachenFormComponent: Name der BI*, Anliegen*, Ansprechpartner,
  Telefon, E-Mail, Datei-Upload (max 5×5MB, JPG/PNG/WebP/GIF/PDF),
  Consent — Honeypots + Client-Age + Turnstile wie Kontaktformular
- /api/mitmachen: Multipart-Validierung (shared Modul), Forwarding an
  CMS /api/forms/mitmachen/submit-multipart
- internal_component: form-mitmachen registriert
- ~35 neue mitmachen_*-Translation-Keys

Prod braucht BODY_SIZE_LIMIT=30M (adapter-node default 512K) — via
secrets .env.prod deployt.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/translations.ts

@@ -317,6 +317,42 @@ const TRANSLATION_KEYS = [
   "contact_error_consent_required",
   "contact_captcha_pending",
   "contact_server_error_captcha",
+  // Mitmachen form (MitmachenFormComponent.svelte). Generische Submit-/
+  // Server-Error-Texte werden aus contact_* wiederverwendet.
+  "mitmachen_label_title",
+  "mitmachen_placeholder_title",
+  "mitmachen_label_description",
+  "mitmachen_placeholder_description",
+  "mitmachen_label_contact_name",
+  "mitmachen_placeholder_contact_name",
+  "mitmachen_label_phone",
+  "mitmachen_placeholder_phone",
+  "mitmachen_label_email",
+  "mitmachen_placeholder_email",
+  "mitmachen_label_files",
+  "mitmachen_files_help",                 // {{n}}, {{mb}}
+  "mitmachen_files_add",
+  "mitmachen_files_remove",
+  "mitmachen_label_consent",
+  "mitmachen_privacy_note",
+  "mitmachen_submit",
+  "mitmachen_success_title",
+  "mitmachen_success_body",
+  "mitmachen_error_title_min",            // {{n}}
+  "mitmachen_error_title_max",            // {{n}}
+  "mitmachen_error_title_invalid",
+  "mitmachen_error_description_min",      // {{n}}
+  "mitmachen_error_description_max",      // {{n}}
+  "mitmachen_error_description_too_many_links", // {{n}}
+  "mitmachen_error_description_invalid",
+  "mitmachen_error_contact_name_max",     // {{n}}
+  "mitmachen_error_contact_name_invalid",
+  "mitmachen_error_phone_invalid",
+  "mitmachen_error_email_invalid",
+  "mitmachen_error_consent_required",
+  "mitmachen_error_files_count",          // {{n}}
+  "mitmachen_error_files_size",           // {{mb}}
+  "mitmachen_error_files_type",
 ] as const;
 
 export type TranslationKey = (typeof TRANSLATION_KEYS)[number];