refactor(slideshow): watermark look lives only in global settings (+ size)

The watermark look (logo / position / opacity / style) was configurable in three
places — the global Settings tab, the per-event-type preset, and the per-event
card. Consolidate it to ONE: the global Settings -> Slideshow tab. Per-event and
per-event-type now carry only the watermark MODE (inherit / on / off) — the
override structure — and render with the global look.

- New global "Size (% of screen)" control (slideshow_watermark_size, vmin-based)
  so the logo can be scaled; resolved server-side into the watermark payload and
  applied to the kiosk <img>.
- Backend slideshowSettings resolves the whole look from app_settings always;
  per-event show_watermark only toggles enabled. adminEvents PATCH + type-preset
  seeding no longer accept/seed per-event look fields; unused enums removed.
- Frontend SlideshowStyle drops the look fields (mode only); SlideshowStyleFields
  watermark section is a single mode select with a "configured under Settings"
  hint; SlideshowSettingsCard + Event type cleaned up.
- en/de: watermarkSizeLabel + watermarkModeHint.

(events.show_watermark_{source,position,opacity,style} columns from migration
138 are left in place but inert — the look is global now.)

backend/src/routes/adminEvents.js

@@ -697,10 +697,6 @@ router.post('/', adminAuth, requirePermission('events.create'), [
             show_transition: oneOf(preset.transition, SLIDESHOW_TRANSITIONS),
             show_transition_ms: intP(preset.transition_ms, 100, 5000),
             show_watermark: watermarkSeed,
-            show_watermark_source: oneOf(preset.watermark_source, SLIDESHOW_WATERMARK_SOURCES),
-            show_watermark_position: oneOf(preset.watermark_position, SLIDESHOW_WATERMARK_POSITIONS),
-            show_watermark_opacity: intP(preset.watermark_opacity, 0, 100),
-            show_watermark_style: oneOf(preset.watermark_style, SLIDESHOW_WATERMARK_STYLES),
             show_colorfilter: oneOf(preset.colorfilter, SLIDESHOW_COLORFILTERS),
           };
           // Only carry through fields the preset actually set.
@@ -1908,12 +1904,9 @@ router.post('/:id/toggle-status', adminAuth, requirePermission('events.edit'), r
 const SLIDESHOW_TRANSITIONS = ['crossfade', 'cut', 'slide', 'kenburns', 'dipwhite', 'dipblack'];
 // Allowed per-slide color filters.
 const SLIDESHOW_COLORFILTERS = ['none', 'bw', 'sepia', 'warm', 'cool', 'vignette'];
-// Allowed watermark logo sources + corners.
-const SLIDESHOW_WATERMARK_SOURCES = ['logo', 'logo_dark', 'favicon', 'event'];
-const SLIDESHOW_WATERMARK_POSITIONS = ['top-left', 'top-right', 'bottom-left', 'bottom-right'];
-// 'white' recolors the logo white; 'original' keeps its own colors (for boxed
-// / colored logos that would otherwise whiten into a solid blob).
-const SLIDESHOW_WATERMARK_STYLES = ['white', 'original'];
+// The watermark LOOK (source/position/opacity/style/size) is global-only
+// (app_settings, Settings → Slideshow); events only carry the show_watermark
+// mode (NULL=inherit / true / false), so no per-event look enums live here.
 
 // Build the public slideshow URL for a freshly-minted/existing token.
 async function buildSlideshowUrl(slug, token) {
@@ -2000,10 +1993,6 @@ router.patch('/:id/slideshow', adminAuth, requirePermission('events.edit'), requ
   body('show_transition').optional().isIn(SLIDESHOW_TRANSITIONS),
   body('show_transition_ms').optional().isInt({ min: 100, max: 5000 }),
   body('show_watermark').optional({ nullable: true }),
-  body('show_watermark_source').optional().isIn(SLIDESHOW_WATERMARK_SOURCES),
-  body('show_watermark_position').optional().isIn(SLIDESHOW_WATERMARK_POSITIONS),
-  body('show_watermark_opacity').optional().isInt({ min: 0, max: 100 }),
-  body('show_watermark_style').optional().isIn(SLIDESHOW_WATERMARK_STYLES),
   body('show_colorfilter').optional().isIn(SLIDESHOW_COLORFILTERS)
 ], async (req, res) => {
   try {
@@ -2028,10 +2017,6 @@ router.patch('/:id/slideshow', adminAuth, requirePermission('events.edit'), requ
         ? null
         : formatBoolean(parseBooleanInput(req.body.show_watermark, false));
     }
-    if (req.body.show_watermark_source !== undefined) updates.show_watermark_source = req.body.show_watermark_source;
-    if (req.body.show_watermark_position !== undefined) updates.show_watermark_position = req.body.show_watermark_position;
-    if (req.body.show_watermark_opacity !== undefined) updates.show_watermark_opacity = parseInt(req.body.show_watermark_opacity, 10);
-    if (req.body.show_watermark_style !== undefined) updates.show_watermark_style = req.body.show_watermark_style;
     if (req.body.show_colorfilter !== undefined) updates.show_colorfilter = req.body.show_colorfilter;
 
     // Knex throws on an empty update; only write if something changed.
@@ -2043,11 +2028,7 @@ router.patch('/:id/slideshow', adminAuth, requirePermission('events.edit'), requ
       show_interval_ms: updates.show_interval_ms ?? event.show_interval_ms ?? 5000,
       show_transition: updates.show_transition ?? event.show_transition ?? 'crossfade',
       show_transition_ms: updates.show_transition_ms ?? event.show_transition_ms ?? 800,
-      show_watermark: updates.show_watermark ?? event.show_watermark ?? false,
-      show_watermark_source: updates.show_watermark_source ?? event.show_watermark_source ?? 'logo',
-      show_watermark_position: updates.show_watermark_position ?? event.show_watermark_position ?? 'bottom-right',
-      show_watermark_opacity: updates.show_watermark_opacity ?? event.show_watermark_opacity ?? 60,
-      show_watermark_style: updates.show_watermark_style ?? event.show_watermark_style ?? 'white',
+      show_watermark: updates.show_watermark ?? event.show_watermark ?? null,
       show_colorfilter: updates.show_colorfilter ?? event.show_colorfilter ?? 'none'
     });
   } catch (error) {

backend/src/routes/adminSettings.js

@@ -317,6 +317,10 @@ router.put('/slideshow', adminAuth, requirePermission('settings.edit'), async (r
       const v = ['white', 'original'].includes(req.body.slideshow_watermark_style) ? req.body.slideshow_watermark_style : 'white';
       push('slideshow_watermark_style', v);
     }
+    if (has('slideshow_watermark_size')) {
+      const n = Math.min(40, Math.max(3, Math.round(Number(req.body.slideshow_watermark_size) || 12)));
+      push('slideshow_watermark_size', n);
+    }
 
     for (const u of updates) {
       await upsertAppSetting(u.setting_key, u.setting_value, u.setting_type);

backend/src/routes/gallery.js

@@ -264,9 +264,10 @@ async function resolveSlideshow(slug, token) {
 // from the chosen source so the kiosk renders it without knowing about
 // branding/event internals; null url = nothing to overlay.
 async function slideshowSettings(event) {
-  // Watermark cascade: per-event `show_watermark` overrides the global default,
-  // NULL inherits it. When inheriting, the source/position/opacity also come
-  // from the global settings; when overriding, from the event's own columns.
+  // Watermark: the LOOK (logo/position/opacity/style/size) is configured ONCE
+  // globally (Settings → Slideshow); it is NOT duplicated per event. The only
+  // per-event control is whether the watermark shows: `show_watermark` NULL
+  // inherits the global enabled flag, true/false force it on/off.
   const wm = event.show_watermark;
   const inherit = (wm === null || wm === undefined);
   const enabled = inherit
@@ -274,18 +275,11 @@ async function slideshowSettings(event) {
     : (wm === true || wm === 1 || wm === '1');
   let watermark = null;
   if (enabled) {
-    const source = inherit
-      ? (await getAppSetting('slideshow_watermark_source', 'logo'))
-      : (event.show_watermark_source || 'logo');
-    const position = inherit
-      ? (await getAppSetting('slideshow_watermark_position', 'bottom-right'))
-      : (event.show_watermark_position || 'bottom-right');
-    const opacity = inherit
-      ? (await getAppSetting('slideshow_watermark_opacity', 60))
-      : (event.show_watermark_opacity ?? 60);
-    const style = inherit
-      ? (await getAppSetting('slideshow_watermark_style', 'white'))
-      : (event.show_watermark_style || 'white');
+    const source = await getAppSetting('slideshow_watermark_source', 'logo');
+    const position = await getAppSetting('slideshow_watermark_position', 'bottom-right');
+    const opacity = await getAppSetting('slideshow_watermark_opacity', 60);
+    const style = await getAppSetting('slideshow_watermark_style', 'white');
+    const size = await getAppSetting('slideshow_watermark_size', 12);
     // Resolve the chosen logo to a URL. Branding assets come from settings;
     // the event source uses the event's own hero logo.
     let url;
@@ -299,7 +293,7 @@ async function slideshowSettings(event) {
       url = await getAppSetting('branding_logo_url', null);
     }
     if (url) {
-      watermark = { url, position: position || 'bottom-right', opacity: opacity ?? 60, style: style || 'white' };
+      watermark = { url, position: position || 'bottom-right', opacity: opacity ?? 60, style: style || 'white', size: size ?? 12 };
     }
   }
   return {