fix(plex): a Surprise card whose watched toggle failed is put back

Deep-review round 3 of bug hunt 2026-09 — the sibling of the collection page's fix (d349bc90).
Discover's toggle flips the Surprise draw too, but its resync refetches only the rails (refetching the
draw would reshuffle it under the viewer), so a refused toggle stayed flipped there. toggleWatchState
takes an optional `revert`, called on a failure only; the queryKeys comment that claimed the watched
toggle invalidates the Surprise key now says what the code does.

frontend/src/components/PlexDiscover.tsx

@@ -105,6 +105,14 @@ export default function PlexDiscover({
 
   // Watched toggle over BOTH this surface's cache (so a rail card flips instantly) and the surprise
   // list, then persist + resync via the shared helper — invalidating the grid too so it agrees.
+  const setSurpriseStatus = (card: PlexCard, status: PlexCard["status"]) =>
+    qc.setQueryData<{ scope: string; items: PlexCard[] }>(
+      qk.plexSurprise(scope, lang, filters),
+      (old) =>
+        old
+          ? { ...old, items: old.items.map((c) => (c.id === card.id ? { ...c, status } : c)) }
+          : old,
+    );
   const toggleWatched = (card: PlexCard) =>
     toggleWatchState(card, t, {
       optimistic: (next) => {
@@ -122,11 +130,11 @@ export default function PlexDiscover({
               }
             : old,
         );
-        qc.setQueryData<{ scope: string; items: PlexCard[] }>(
-          qk.plexSurprise(scope, lang, filters),
-          (old) => (old ? { ...old, items: flip(old.items) } : old),
-        );
+        setSurpriseStatus(card, next);
       },
+      // The resync below leaves the Surprise rail alone (a refetch would reshuffle it), so a failed
+      // toggle was left flipped there — put back in place instead.
+      revert: () => setSurpriseStatus(card, card.status),
       resync: () => {
         qc.invalidateQueries({ queryKey: qk.plexDiscover() });
         qc.invalidateQueries({ queryKey: qk.plexLibrary() });