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/services/events.service.ts

@@ -158,6 +158,8 @@ export const eventsService = {
       show_transition_ms?: number;
       show_watermark?: boolean | null;
       show_colorfilter?: string;
+      show_order?: string;
+      show_category_id?: number | null;
     }
   ): Promise<Record<string, unknown>> {
     const response = await api.patch(`/admin/events/${id}/slideshow`, settings);

frontend/src/services/slideshow.service.ts

@@ -18,6 +18,9 @@ export const SLIDESHOW_WATERMARK_STYLES: SlideshowWatermarkStyle[] = ['white', '
 // (admin Settings → Slideshow); 'on'/'off' = explicit override.
 export type SlideshowWatermarkMode = 'inherit' | 'on' | 'off';
 export const SLIDESHOW_WATERMARK_MODES: SlideshowWatermarkMode[] = ['inherit', 'on', 'off'];
+// Play order (#202): 'chronological' = upload order; 'random' = client shuffle.
+export type SlideshowOrder = 'chronological' | 'random';
+export const SLIDESHOW_ORDERS: SlideshowOrder[] = ['chronological', 'random'];
 
 export const SLIDESHOW_TRANSITIONS: SlideshowTransition[] = ['crossfade', 'cut', 'slide', 'kenburns', 'dipwhite', 'dipblack'];
 export const SLIDESHOW_COLORFILTERS: SlideshowColorFilter[] = ['none', 'bw', 'sepia', 'warm', 'cool', 'vignette'];
@@ -37,6 +40,9 @@ export interface SlideshowStyle {
   transition_ms: number;
   watermark: SlideshowWatermarkMode;
   colorfilter: SlideshowColorFilter;
+  // Play order + optional category filter (#202). category_id null = all photos.
+  order: SlideshowOrder;
+  category_id: number | null;
 }
 
 export const DEFAULT_SLIDESHOW_STYLE: SlideshowStyle = {
@@ -45,6 +51,8 @@ export const DEFAULT_SLIDESHOW_STYLE: SlideshowStyle = {
   transition_ms: 800,
   watermark: 'inherit',
   colorfilter: 'none',
+  order: 'chronological',
+  category_id: null,
 };
 
 // Global slideshow defaults (admin Settings → Slideshow). The single source of
@@ -81,6 +89,10 @@ export interface SlideshowSettings {
   transition: SlideshowTransition;
   transition_ms: number;
   colorfilter: SlideshowColorFilter;
+  // Play order the kiosk applies (#202): 'random' shuffles client-side so
+  // live-appended uploads keep working. The category filter is enforced
+  // server-side, so it isn't echoed here.
+  order: SlideshowOrder;
   fit: SlideshowFit;
   watermark: SlideshowWatermark | null;
 }