fix(plex): address code-review findings (F1-F3, F5, F9)

- F1: refresh grid after playback/collection-edit — invalidate the renamed
  ["plex-library"] query key in PlexPlayer + PlexCollectionEditor (was stale
  ["plex-browse"], a dead no-op)
- F2: exclude hidden movies from facet counts/bounds — the movie facet base now
  always joins watch-state and mirrors unified_library's status branches, so
  facets match the visible grid exactly
- F3: show_detail/item_detail live-Plex enrichment degrades instead of 500ing —
  broaden the best-effort block to except Exception
- F5: don't re-run the heavy facet computation on sort-direction toggles — key
  the facets query only on fields plexFacets actually sends
- F9: optimistic watched-toggle also updates the search Episodes section and
  gives toggleEpisodeWatched an optimistic path (shared optimisticStatus helper)

frontend/src/components/PlexSidebar.tsx

@@ -64,8 +64,12 @@ export default function PlexSidebar({
   }
   // Facet values (genres/ratings/bounds) for the current scope, ADAPTED to the active filters
   // (faceted search) — so picking one filter narrows what the others offer. Refetches on any change.
+  // Key only on the fields plexFacets actually sends — sortDir/collectionTitle don't reach /facets,
+  // so keying on the whole `filters` object would re-run the heavy facet computation on every
+  // sort-direction toggle (or chip-title change) for an identical request.
+  const { sortDir: _sd, collectionTitle: _ct, ...facetKey } = filters;
   const facetsQ = useQuery({
-    queryKey: ["plex-facets", scope, show, filters],
+    queryKey: ["plex-facets", scope, show, facetKey],
     queryFn: () => api.plexFacets(scope, filters, show),
     enabled: !!scope,
     placeholderData: (prev) => prev, // keep the old chips visible during the refetch (no flicker)