feat(slideshow): gate behind a feature flag + move globals to a Settings tab

- New `slideshow` feature flag (backend KNOWN_FLAGS/DEFAULT_FLAGS, frontend
  FeatureKey + context default, a toggle card under Settings -> Features -> Core).
  Default off; strictly opt-in.
- Move the global watermark defaults off the Event Types page into a dedicated
  Settings -> Slideshow tab (new SlideshowSettingsPage), shown only when the flag
  is on.
- Gate the per-event Live Slideshow card and the per-event-type preset section
  behind the flag too (and stop writing a type preset when it's off).
- en/de strings for the feature card + settings tab.

frontend/src/contexts/FeatureFlagsContext.tsx

@@ -62,6 +62,8 @@ export const DEFAULT_FLAGS: FeatureFlags = {
   projects: false,
   // WhatsApp Business API delivery channel (migration 136, #640D).
   whatsapp: false,
+  // Live Slideshow ("Diashow") — opt-in; gates all slideshow admin UI.
+  slideshow: false,
 };
 
 export const FEATURE_FLAGS_QUERY_KEY = ['feature-flags'] as const;