fix(cms): nl/pt/ru i18n + gate external_url in public response

Two follow-ups to PR #372 (external-URL toggle for imprint /
privacy CMS pages):

1. **i18n.** PR #372 added 6 new `cms.*` keys to the en + de
   locales but the project ships 5 locales total. Adds the missing
   nl / pt / ru translations so the admin CMS page renders in the
   active language for those users instead of falling back to
   English literals next to the German/Dutch/Portuguese/Russian
   surrounding strings.

2. **API shape.** `publicCMS.js` returned `external_url`
   unconditionally — even when `use_external_url` is false the URL
   value was still emitted in the public response. The frontend
   correctly gated on both flags so it worked, but the API surface
   was leaking a value the admin had explicitly disabled. The
   value still lives in the DB (so the toggle can be flipped back
   on without losing it), but the public endpoint now returns
   `null` whenever the toggle is off.

   Note: kept the existing `logo_url` shape unchanged. Its semantics
   are different — null means "fall back to global branding" and
   consumers rely on always having the field, so emitting it
   unconditionally is intentional there.

No frontend change needed: both `GalleryLayout` and `LegalPage`
already gate on `use_external_url && external_url`, so the
short-circuit handles `external_url: null` correctly.

frontend/src/i18n/locales/nl.json

@@ -1753,7 +1753,13 @@
     "lastUpdated": "Laatst bijgewerkt:",
     "impressum": "Colofon",
     "datenschutz": "Privacybeleid",
-    "pageUpdated": "Pagina succesvol bijgewerkt"
+    "pageUpdated": "Pagina succesvol bijgewerkt",
+    "useExternalUrl": "Externe URL gebruiken",
+    "useExternalUrlHelp": "Bezoekers worden doorgestuurd naar een externe pagina in plaats van de interne inhoud te zien. De interne titel en inhoud blijven als fallback opgeslagen.",
+    "externalUrl": "Externe URL",
+    "externalUrlPlaceholder": "https://example.com/impressum",
+    "externalUrlInvalid": "Moet een geldige https://-URL zijn",
+    "externalUrlActive": "Externe URL is actief — interne inhoud blijft bewaard maar wordt niet aan bezoekers getoond."
   },
   "eventTypes": {
     "title": "Evenementtypes",

frontend/src/i18n/locales/pt.json

@@ -1753,7 +1753,13 @@
     "lastUpdated": "Última atualização:",
     "impressum": "Aviso Legal",
     "datenschutz": "Política de Privacidade",
-    "pageUpdated": "Página atualizada com sucesso"
+    "pageUpdated": "Página atualizada com sucesso",
+    "useExternalUrl": "Usar URL externa",
+    "useExternalUrlHelp": "Redirecione os visitantes para uma página externa em vez de mostrar o conteúdo interno. O título e o conteúdo internos permanecem salvos como fallback.",
+    "externalUrl": "URL externa",
+    "externalUrlPlaceholder": "https://example.com/impressum",
+    "externalUrlInvalid": "Deve ser uma URL https:// válida",
+    "externalUrlActive": "URL externa está ativa — o conteúdo interno é preservado, mas não é exibido aos visitantes."
   },
   "eventTypes": {
     "title": "Tipos de Evento",

frontend/src/i18n/locales/ru.json

@@ -1753,7 +1753,13 @@
     "lastUpdated": "Последнее обновление:",
     "impressum": "Правовая информация",
     "datenschutz": "Политика конфиденциальности",
-    "pageUpdated": "Страница успешно обновлена"
+    "pageUpdated": "Страница успешно обновлена",
+    "useExternalUrl": "Использовать внешний URL",
+    "useExternalUrlHelp": "Перенаправлять посетителей на внешнюю страницу вместо показа внутреннего содержимого. Внутренний заголовок и содержимое остаются сохранёнными как резервная копия.",
+    "externalUrl": "Внешний URL",
+    "externalUrlPlaceholder": "https://example.com/impressum",
+    "externalUrlInvalid": "Должен быть действительный URL вида https://",
+    "externalUrlActive": "Внешний URL активен — внутреннее содержимое сохранено, но не отображается посетителям."
   },
   "eventTypes": {
     "title": "Типы событий",