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/lib/plexWatch.ts

@@ -6,16 +6,20 @@ import { notifyActionError } from "./actionErrors";
 // updater, call the API, surface a failure (the click used to look dead on 403/404/429), then
 // `resync` (invalidate) — which corrects the optimistic flip to truth either way. Shared by the
 // browse grid and the person page, each of which owns a different cache shape.
+//
+// `revert` is for a cache `resync` must NOT refetch — a random draw (Discover's Surprise rail), which
+// a refetch would reshuffle under the viewer. It is called on a failure only, to put the card back.
 export async function toggleWatchState(
   card: PlexCard,
   t: TFunction,
-  handlers: { optimistic: (next: string) => void; resync: () => void },
+  handlers: { optimistic: (next: string) => void; resync: () => void; revert?: () => void },
 ) {
   const next = card.status === "watched" ? "new" : "watched";
   handlers.optimistic(next);
   try {
     await api.plexSetState(card.id, next);
   } catch (e) {
+    handlers.revert?.();
     notifyActionError(e, t("plex.watchedFailed"));
   }
   handlers.resync();

frontend/src/lib/queryKeys.ts

@@ -117,8 +117,9 @@ export const qk = {
   // refetches it alone.
   plexDiscover: (...rest: unknown[]) => ["plex-discover", ...rest],
   // Variadic like plexDiscover so it can key by scope + lang + the active filters (lang because a genre
-  // filter narrows against the language-specific genre column, like the grid); `plexSurprise()` (no args)
-  // stays a prefix for the reshuffle/watched-toggle invalidations.
+  // filter narrows against the language-specific genre column, like the grid). The ↻ reshuffle is the
+  // query's own refetch, and nothing invalidates this key — deliberately: a watched toggle flips the
+  // card in place (and reverts it on a failure) rather than reshuffle the rail under the viewer.
   plexSurprise: (...rest: unknown[]) => ["plex-surprise", ...rest],
   plexSuggest: (...rest: unknown[]) => ["plex-suggest", ...rest],
   // Optional trailing `lang` (E5 S6): a language switch remounts the detail with a new key so HU/EN