perf(slideshow): cache global settings to cut /state DB reads (PR #646 review)

Each /state poll fired ~10 getAppSetting reads to resolve the watermark/fit;
a leaked link x N tabs amplified that linearly (review concern 2). Add a
5s-TTL cached bundle (utils/slideshowGlobals) for the global slideshow_* +
branding-logo settings, invalidated on PUT /admin/settings/slideshow so admin
live-edit stays instant. slideshowSettings now does ~2 reads per poll (event
row + photo count) on a cache hit. Also documents the frontend
optimistic-default nit.

frontend/src/components/admin/SlideshowGlobalDefaultsCard.tsx

@@ -23,6 +23,9 @@ import {
 } from '../../services/slideshow.service';
 import { WatermarkSourcePicker } from './WatermarkSourcePicker';
 
+// Optimistic form state shown until the GET resolves; mirrors the backend
+// defaults so the controls don't flicker on load (backend is the source of
+// truth — these are just the pre-fetch placeholder).
 const DEFAULTS: SlideshowGlobalDefaults = {
   slideshow_fit: 'cover',
   slideshow_interval_ms: 5000,