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.

backend/src/routes/adminFeatureFlags.js

@@ -84,6 +84,10 @@ const KNOWN_FLAGS = [
   // opt-in — operators must register a Meta-approved template before turning
   // it on. Independent of email; both can fire on the same event.
   'whatsapp',
+  // Live Slideshow ("Diashow") — the per-event fullscreen kiosk link + its
+  // per-event-type presets and global watermark defaults tab. Strictly opt-in;
+  // gates all slideshow admin UI (per-event card, type preset, settings tab).
+  'slideshow',
 ];
 
 // Spec defaults for any flag missing from the DB (e.g. a row added by a
@@ -112,6 +116,7 @@ const DEFAULT_FLAGS = {
   expenses: false,
   projects: false,
   whatsapp: false,
+  slideshow: false,
 };
 
 async function readAllFlags() {

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;

frontend/src/features/settings/tabs/FeaturesTab.tsx

@@ -22,6 +22,7 @@ import {
   ScanLine,
   Wallet,
   FolderKanban,
+  MonitorPlay,
 } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
 import { Button, Card } from '../../../components/common';
@@ -116,6 +117,21 @@ export const FeaturesTab: React.FC = () => {
               "Galleries are the foundation of PicPeak and can't be turned off.",
             )}
           />
+
+          <FeatureCard
+            icon={MonitorPlay}
+            title={t('settings.features.slideshow.title', 'Live Slideshow')}
+            description={t(
+              'settings.features.slideshow.description',
+              'A separate fullscreen "Diashow" link per event for projectors at live events — auto-picks-up new uploads, with per-event-type presets and global watermark defaults under Settings → Slideshow.',
+            )}
+            status="new"
+            statusLabel={statusLabel('new')}
+            sidebarHidden
+            sidebarHiddenLabel={sidebarHiddenLabel}
+            enabled={staged.slideshow}
+            onToggle={(next) => setFlag('slideshow', next)}
+          />
         </Section>
 
         {/* Clients (#354 follow-up). Visual grouping for the CRM-area

frontend/src/i18n/locales/de.json

@@ -1768,6 +1768,10 @@
       "whatsapp": {
         "title": "WhatsApp",
         "description": "Liefert die Gallerie-Bereit-Benachrichtigung zusätzlich zur E-Mail über die WhatsApp Business API. Voraussetzung: Meta-Business-Konto, genehmigte Nachrichtenvorlage und eine Kunden-Telefonnummer am Event. Zugangsdaten unter Einstellungen → WhatsApp konfigurieren."
+      },
+      "slideshow": {
+        "title": "Live-Diashow",
+        "description": "Ein separater Vollbild-„Diashow“-Link pro Event für Beamer bei Live-Events – übernimmt neue Uploads automatisch, mit Voreinstellungen je Event-Typ und globalen Wasserzeichen-Vorgaben unter Einstellungen → Diashow."
       }
     },
     "customerSurface": {
@@ -1815,6 +1819,10 @@
         "hourlyRateHint": "Verrechnungs-Fallback, wenn ein Kunde keinen eigenen Satz hat (Stundenerfassung). In {{currency}}, in Hauptwährungseinheiten. Leer lassen, um einen Satz pro Kunde oder pro Eintrag zu verlangen."
       }
     },
+    "slideshow": {
+      "title": "Diashow",
+      "subtitle": "Globale Vorgaben für die Live-Diashow. Events und Event-Typen können diese überschreiben."
+    },
     "whatsapp": {
       "title": "WhatsApp",
       "subtitle": "Meta-Business-Zugangsdaten konfigurieren, um die Gallerie-Bereit-Benachrichtigung zusätzlich zur E-Mail per WhatsApp zu senden.",

frontend/src/i18n/locales/en.json

@@ -1324,6 +1324,10 @@
       "whatsapp": {
         "title": "WhatsApp",
         "description": "Deliver the gallery-ready notification via WhatsApp Business API in addition to email. Requires a Meta Business Account, an approved message template, and a customer phone number on the event. Configure credentials under Settings → WhatsApp."
+      },
+      "slideshow": {
+        "title": "Live Slideshow",
+        "description": "A separate fullscreen \"Diashow\" link per event for projectors at live events — auto-picks-up new uploads, with per-event-type presets and global watermark defaults under Settings → Slideshow."
       }
     },
     "customerSurface": {
@@ -1371,6 +1375,10 @@
         "hourlyRateHint": "Billing fallback used when a customer has no own rate (hours logging). In {{currency}}, major units. Leave blank to require a per-customer or per-entry rate."
       }
     },
+    "slideshow": {
+      "title": "Slideshow",
+      "subtitle": "Global defaults for the Live Slideshow. Events and event types can override these."
+    },
     "whatsapp": {
       "title": "WhatsApp",
       "subtitle": "Configure Meta Business credentials to deliver the gallery-ready notification via WhatsApp alongside email.",

frontend/src/pages/admin/EventDetailsPage.tsx

@@ -2159,7 +2159,9 @@ export const EventDetailsPage: React.FC = () => {
             </div>
           </Card>
 
-          {/* Live Slideshow ("Diashow") link + live display settings (migration 137) */}
+          {/* Live Slideshow ("Diashow") link + live display settings (migrations 137/138).
+              Gated behind the `slideshow` feature flag. */}
+          {flags.slideshow && (
           <SlideshowSettingsCard
             eventId={event.id}
             slug={event.slug}
@@ -2179,6 +2181,7 @@ export const EventDetailsPage: React.FC = () => {
             }}
             onChanged={() => refetchEvent()}
           />
+          )}
 
           {/* Pre-event reminder override (migration 143). Hidden when
               the reminderEmails master flag is off — the override here

frontend/src/pages/admin/EventTypesPage.tsx

@@ -19,8 +19,8 @@ import { Button, Input, Card, Loading } from '../../components/common';
 import { eventTypesService, EventType, CreateEventTypeData, UpdateEventTypeData } from '../../services/eventTypes.service';
 import { GALLERY_THEME_PRESETS } from '../../types/theme.types';
 import { SlideshowStyleFields } from '../../components/admin/SlideshowStyleFields';
-import { SlideshowGlobalDefaultsCard } from '../../components/admin/SlideshowGlobalDefaultsCard';
 import { DEFAULT_SLIDESHOW_STYLE, type SlideshowStyle } from '../../services/slideshow.service';
+import { useFeatureEnabled } from '../../contexts/FeatureFlagsContext';
 
 // Parse a type's stored slideshow_preset (JSON string | object | null) into a
 // full SlideshowStyle, falling back to defaults for any missing keys.
@@ -152,9 +152,6 @@ export const EventTypesPage: React.FC = () => {
         </div>
       </div>
 
-      {/* Global Live Slideshow watermark default (per-event/type can override) */}
-      <SlideshowGlobalDefaultsCard />
-
       {/* Filters */}
       <Card className="mb-6">
         <div className="p-4 flex flex-col sm:flex-row gap-4">
@@ -339,6 +336,7 @@ const EventTypeModal: React.FC<EventTypeModalProps> = ({
 }) => {
   const { t } = useTranslation();
   const isEditing = !!eventType;
+  const slideshowEnabled = useFeatureEnabled('slideshow');
 
   const [form, setForm] = useState<CreateEventTypeData>({
     name: eventType?.name || '',
@@ -382,10 +380,10 @@ const EventTypeModal: React.FC<EventTypeModalProps> = ({
       if (form.emoji !== eventType?.emoji) updates.emoji = form.emoji;
       if (form.theme_preset !== eventType?.theme_preset) updates.theme_preset = form.theme_preset;
       const originalStyle = JSON.stringify(parseSlideshowPreset(eventType?.slideshow_preset));
-      if (JSON.stringify(slideshowStyle) !== originalStyle) updates.slideshow_preset = slideshowStyle;
+      if (slideshowEnabled && JSON.stringify(slideshowStyle) !== originalStyle) updates.slideshow_preset = slideshowStyle;
       onSubmit(updates);
     } else {
-      onSubmit({ ...form, slideshow_preset: slideshowStyle });
+      onSubmit({ ...form, ...(slideshowEnabled ? { slideshow_preset: slideshowStyle } : {}) });
     }
   };
 
@@ -488,7 +486,9 @@ const EventTypeModal: React.FC<EventTypeModalProps> = ({
 
               {/* Live Slideshow preset (migration 138). New events of this type
                   inherit these slideshow defaults; admins can still override
-                  per event on the event detail page. */}
+                  per event on the event detail page. Gated behind the
+                  `slideshow` feature flag. */}
+              {slideshowEnabled && (
               <div className="pt-2 border-t border-neutral-200 dark:border-neutral-700">
                 <label className="block text-sm font-medium text-neutral-700 dark:text-neutral-300 mb-1">
                   {t('eventTypes.form.slideshowPreset', 'Slideshow preset')}
@@ -498,6 +498,7 @@ const EventTypeModal: React.FC<EventTypeModalProps> = ({
                 </p>
                 <SlideshowStyleFields value={slideshowStyle} onChange={setSlideshowStyle} />
               </div>
+              )}
 
               {/* Active toggle for editing */}
               {isEditing && (

frontend/src/pages/admin/SettingsPage.tsx

@@ -46,6 +46,7 @@ import {
 import { EmailConfigPage } from './EmailConfigPage';
 import { BrandingPage } from './BrandingPage';
 import { EventTypesPage } from './EventTypesPage';
+import { SlideshowSettingsPage } from './SlideshowSettingsPage';
 import { BackupManagement } from './BackupManagement';
 import { CMSPage } from './CMSPage';
 // CRM (#TBD)
@@ -54,7 +55,7 @@ import { CrmSettingsPage } from './settings/CrmSettingsPage';
 import { ReminderTemplatesPage } from './settings/ReminderTemplatesPage';
 import { BlockLibraryPage } from './contracts/BlockLibraryPage';
 import { useFeatureFlags } from '../../contexts/FeatureFlagsContext';
-import { Briefcase, Receipt, ScrollText, Mail, Landmark, Smartphone } from 'lucide-react';
+import { Briefcase, Receipt, ScrollText, Mail, Landmark, Smartphone, MonitorPlay } from 'lucide-react';
 
 // Tab keys driving the inner-nav. Must include every key used in
 // `navGroups` below and in the switch at the bottom of the component.
@@ -85,7 +86,8 @@ type TabType =
   | 'contracts'
   | 'reminderTemplates'
   | 'accounting'
-  | 'whatsapp';
+  | 'whatsapp'
+  | 'slideshow';
 
 interface NavItem {
   key: TabType;
@@ -106,6 +108,7 @@ const ALL_TAB_KEYS: TabType[] = [
   'apiTokens', 'webhooks',
   'status', 'analytics', 'backup',
   'businessProfile', 'crm', 'contracts', 'reminderTemplates', 'accounting', 'whatsapp',
+  'slideshow',
 ];
 
 function isValidTab(value: string | null): value is TabType {
@@ -203,12 +206,13 @@ export const SettingsPage: React.FC = () => {
       reminderTemplates: !flags.reminderEmails,
       accounting: !flags.accounting,
       whatsapp: !flags.whatsapp,
+      slideshow: !flags.slideshow,
     };
     if (gatedOff[activeTab]) {
       setActiveTab('features');
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [flagsLoading, flags.quotes, flags.bills, flags.contracts, flags.reminderEmails, flags.accounting, flags.whatsapp, activeTab]);
+  }, [flagsLoading, flags.quotes, flags.bills, flags.contracts, flags.reminderEmails, flags.accounting, flags.whatsapp, flags.slideshow, activeTab]);
 
   if (isLoading) {
     return (
@@ -240,6 +244,9 @@ export const SettingsPage: React.FC = () => {
         { key: 'thumbnails', label: t('settings.thumbnails.title', 'Thumbnails'),  icon: ImageIcon },
         { key: 'styling',    label: t('settings.styling.title',    'Custom CSS'),  icon: Code },
         { key: 'cms',        label: t('settings.cms.title',        'CMS Pages'),   icon: FileText },
+        ...(flags.slideshow
+          ? [{ key: 'slideshow' as const, label: t('settings.slideshow.title', 'Slideshow'), icon: MonitorPlay }]
+          : []),
       ],
     },
     {
@@ -433,6 +440,7 @@ export const SettingsPage: React.FC = () => {
           )}
 
           {activeTab === 'eventTypes' && <EventTypesPage />}
+          {activeTab === 'slideshow' && <SlideshowSettingsPage />}
           {activeTab === 'branding' && <BrandingPage />}
           {activeTab === 'cms' && <CMSPage />}
           {activeTab === 'email' && <EmailConfigPage />}

frontend/src/pages/admin/SlideshowSettingsPage.tsx

@@ -0,0 +1,28 @@
+/**
+ * Settings → Slideshow tab. Top-level (global) Live Slideshow settings.
+ * Currently the global watermark default (source/position/opacity/style) that
+ * every event inherits unless it overrides. Gated behind the `slideshow`
+ * feature flag (see SettingsPage nav).
+ */
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+import { SlideshowGlobalDefaultsCard } from '../../components/admin/SlideshowGlobalDefaultsCard';
+
+export const SlideshowSettingsPage: React.FC = () => {
+  const { t } = useTranslation();
+  return (
+    <div className="space-y-6">
+      <div>
+        <h2 className="text-lg font-semibold text-neutral-900 dark:text-neutral-100">
+          {t('settings.slideshow.title', 'Slideshow')}
+        </h2>
+        <p className="text-sm text-neutral-600 dark:text-neutral-400 mt-1">
+          {t('settings.slideshow.subtitle', 'Global defaults for the Live Slideshow. Events and event types can override these.')}
+        </p>
+      </div>
+      <SlideshowGlobalDefaultsCard />
+    </div>
+  );
+};
+
+export default SlideshowSettingsPage;

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>;