feat(slideshow): per-event play order + category filter (#202)

The Live Slideshow already covers the core of #202 (fullscreen kiosk,
live-appending new uploads, timing/transitions/watermark, per-event
opt-in via the share link). This adds the two customization dimensions
the reporter also asked for:

- **Play order** (show_order): 'chronological' (upload order, default) or
  'random' — the client shuffles the initial set (Fisher-Yates) so
  live-appended uploads keep working.
- **Category filter** (show_category_id): restrict the slideshow to a
  single photo category (NULL = all photos, default). Enforced
  server-side on the slideshow /photos access and mirrored in the
  /session + /state photo_count, so the kiosk viewer can't widen the set.

Per-event enable/disable (default off) is unchanged — it's the existing
'Generate/Disable slideshow link' flow (no token = no slideshow).

- Migration 158: show_order (default 'chronological') + show_category_id.
- Admin: Play-order dropdown + category picker in the Live Slideshow card
  (picker hidden for events without categories); EN + DE i18n.
- Verified: migration (SQLite + PG); live API (category filter → 3/2/5
  photos + matching count; order propagates) and the running kiosk
  requests exactly the filtered set; tsc clean, 136 backend tests pass.

frontend/src/pages/gallery/SlideshowPage.tsx

@@ -13,6 +13,7 @@ const DEFAULT_SETTINGS: SlideshowSettings = {
   transition: 'crossfade',
   transition_ms: 800,
   colorfilter: 'none',
+  order: 'chronological',
   fit: 'cover',
   watermark: null,
 };
@@ -65,6 +66,17 @@ function watermarkCorner(position: string): React.CSSProperties {
 
 type Phase = 'splash' | 'running' | 'ended';
 
+// Fisher–Yates shuffle for the 'random' play order (#202). Used once on the
+// initial photo set; live-appended uploads keep landing at the end.
+function shuffle<T>(arr: T[]): T[] {
+  const a = [...arr];
+  for (let i = a.length - 1; i > 0; i--) {
+    const j = Math.floor(Math.random() * (i + 1));
+    [a[i], a[j]] = [a[j], a[i]];
+  }
+  return a;
+}
+
 export function SlideshowPage() {
   const { slug = '', token = '' } = useParams<{ slug: string; token: string }>();
   const { t } = useTranslation();
@@ -159,13 +171,15 @@ export function SlideshowPage() {
       storeGalleryToken(slug, session.token);
       setActiveGallerySlug(slug);
       setEventName(session.event.event_name || '');
-      setSettings(session.settings || DEFAULT_SETTINGS);
+      const settings = session.settings || DEFAULT_SETTINGS;
+      setSettings(settings);
 
       // Load the list and DECODE the first slide (and the next) before we flip
       // to running, so playback starts on an already-rasterised image instead
       // of struggling on the first transition.
       const data = await galleryService.getGalleryPhotos(slug);
-      const list = data.photos || [];
+      // 'random' shuffles the initial set once; new uploads still append (#202).
+      const list = settings.order === 'random' ? shuffle(data.photos || []) : (data.photos || []);
       setPhotos(list);
       await preloadDecode(list[0]);
       void preloadDecode(list[1]);