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/components/admin/SlideshowSettingsCard.tsx

@@ -16,11 +16,13 @@
  * POST .../slideshow/{generate,disable}.
  */
 import React, { useState } from 'react';
+import { useQuery } from '@tanstack/react-query';
 import { useTranslation } from 'react-i18next';
 import { toast } from 'react-toastify';
 import { MonitorPlay, Copy, CheckCircle, RotateCw, Trash2, Save } from 'lucide-react';
 import { Button, Card } from '../common';
 import { eventsService } from '../../services/events.service';
+import { categoriesService } from '../../services/categories.service';
 import { DEFAULT_SLIDESHOW_STYLE, type SlideshowStyle } from '../../services/slideshow.service';
 import { SlideshowStyleFields } from './SlideshowStyleFields';
 
@@ -35,6 +37,8 @@ export interface SlideshowSettingsCardProps {
     show_transition_ms?: number;
     show_watermark?: boolean | null;
     show_colorfilter?: string;
+    show_order?: string;
+    show_category_id?: number | null;
   };
   onChanged?: () => void;
 }
@@ -52,6 +56,8 @@ function styleFromInitial(initial: SlideshowSettingsCardProps['initial']): Slide
     transition_ms: initial.show_transition_ms ?? DEFAULT_SLIDESHOW_STYLE.transition_ms,
     watermark: watermarkMode(initial.show_watermark),
     colorfilter: (initial.show_colorfilter as SlideshowStyle['colorfilter']) ?? DEFAULT_SLIDESHOW_STYLE.colorfilter,
+    order: (initial.show_order as SlideshowStyle['order']) ?? DEFAULT_SLIDESHOW_STYLE.order,
+    category_id: initial.show_category_id ?? null,
   };
 }
 
@@ -68,6 +74,14 @@ export const SlideshowSettingsCard: React.FC<SlideshowSettingsCardProps> = ({
 
   const link = token ? `${window.location.origin}/gallery/${slug}/show/${token}` : '';
 
+  // Event categories for the slideshow content filter (#202). Global + this
+  // event's own categories; empty for events without any → picker hides.
+  const { data: categories = [] } = useQuery({
+    queryKey: ['event-categories', eventId],
+    queryFn: () => categoriesService.getEventCategories(eventId),
+    staleTime: 60_000,
+  });
+
   const generate = async () => {
     setBusy(true);
     try {
@@ -129,6 +143,8 @@ export const SlideshowSettingsCard: React.FC<SlideshowSettingsCardProps> = ({
         // is global-only (Settings → Slideshow); we only send the mode here.
         show_watermark: style.watermark === 'inherit' ? null : style.watermark === 'on',
         show_colorfilter: style.colorfilter,
+        show_order: style.order,
+        show_category_id: style.category_id,
       });
       toast.success(t('slideshow.settingsSaved', 'Slideshow settings saved'));
       onChanged?.();
@@ -208,7 +224,7 @@ export const SlideshowSettingsCard: React.FC<SlideshowSettingsCardProps> = ({
 
             {/* Live style settings */}
             <div className="pt-2 border-t border-neutral-200 dark:border-neutral-700">
-              <SlideshowStyleFields value={style} onChange={setStyle} />
+              <SlideshowStyleFields value={style} onChange={setStyle} categories={categories} />
             </div>
             <p className="text-xs text-neutral-500 dark:text-neutral-400">
               {t('slideshow.liveHint', 'Changes apply to a running slideshow within a few seconds — no need to regenerate the link.')}