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.)

frontend/src/components/admin/SlideshowGlobalDefaultsCard.tsx

@@ -26,6 +26,7 @@ const DEFAULTS: SlideshowGlobalDefaults = {
   slideshow_watermark_position: 'bottom-right',
   slideshow_watermark_opacity: 60,
   slideshow_watermark_style: 'white',
+  slideshow_watermark_size: 12,
 };
 
 const inputClass =
@@ -47,6 +48,7 @@ export const SlideshowGlobalDefaultsCard: React.FC = () => {
         slideshow_watermark_position: s.slideshow_watermark_position ?? DEFAULTS.slideshow_watermark_position,
         slideshow_watermark_opacity: s.slideshow_watermark_opacity ?? DEFAULTS.slideshow_watermark_opacity,
         slideshow_watermark_style: s.slideshow_watermark_style ?? DEFAULTS.slideshow_watermark_style,
+        slideshow_watermark_size: s.slideshow_watermark_size ?? DEFAULTS.slideshow_watermark_size,
       });
     }).catch(() => { /* keep defaults */ });
     return () => { cancelled = true; };
@@ -142,6 +144,18 @@ export const SlideshowGlobalDefaultsCard: React.FC = () => {
                 ))}
               </select>
             </div>
+            <div>
+              <label className={labelClass}>{t('slideshow.watermarkSizeLabel', 'Size (% of screen)')}</label>
+              <input
+                type="number"
+                min={3}
+                max={40}
+                step={1}
+                value={val.slideshow_watermark_size}
+                onChange={(e) => setVal({ ...val, slideshow_watermark_size: Math.min(40, Math.max(3, parseInt(e.target.value, 10) || 12)) })}
+                className={inputClass}
+              />
+            </div>
             </div>
           </div>
         )}