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/i18n/locales/de.json

@@ -3476,6 +3476,13 @@
       "cool": "Kühl",
       "vignette": "Vignette"
     },
+    "orderLabel": "Reihenfolge",
+    "order": {
+      "chronological": "Chronologisch",
+      "random": "Zufällig (mischen)"
+    },
+    "categoryLabel": "Nur Kategorie zeigen",
+    "categoryAll": "Alle Fotos",
     "watermarkToggle": "Logo-Wasserzeichen anzeigen",
     "watermarkDescription": "Blendet ein weißes, halbtransparentes Logo in einer Ecke ein (wie ein Senderlogo im TV).",
     "watermarkSourceLabel": "Logo",

frontend/src/i18n/locales/en.json

@@ -3604,6 +3604,13 @@
       "cool": "Cool",
       "vignette": "Vignette"
     },
+    "orderLabel": "Play order",
+    "order": {
+      "chronological": "Chronological",
+      "random": "Random (shuffle)"
+    },
+    "categoryLabel": "Show only category",
+    "categoryAll": "All photos",
     "watermarkToggle": "Show logo watermark",
     "watermarkDescription": "Overlay a white, semi-transparent logo in a corner (like a TV station ident).",
     "watermarkSourceLabel": "Logo",