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/services/featureFlags.service.ts

@@ -63,7 +63,10 @@ export type FeatureKey =
   // Strictly opt-in — requires a Meta Business Account, an approved
   // message template, and a Meta access token. Independent of email; both
   // can fire on the same event.
-  | 'whatsapp';
+  | 'whatsapp'
+  // Live Slideshow ("Diashow") — per-event fullscreen kiosk link + presets +
+  // global watermark settings tab. Strictly opt-in; gates all slideshow UI.
+  | 'slideshow';
 
 export type FeatureFlags = Record<FeatureKey, boolean>;