Handle the server's LibraryChanged WebSocket message to invalidate
library-dependent React Query caches when items are added/updated/
removed, so newly added episodes/movies appear without a manual
refresh. Debounced to coalesce a scan's burst of events.
Add useRefreshLibraryOnFocus as a fallback that re-checks on screen
focus (throttled, online-only, skips first focus), wired into home
(mobile + TV) and the library pages.
