feat(plex): apply the sidebar filters to the Discover rails too

Discover now behaves like the Browse grid — the active metadata filters (genre /
rating / year / collection / studio / …) narrow every rail: Continue watching,
Recently added, Surprise me, the genre tiles, and Because you watched. Same
LibraryFilters / meta_filter_conds the grid uses, applied to each rail's movie +
show query (movie duration included); a rail may come back empty, which the client
already hides. Genre-language matches the grid.

Because Discover honours filters itself, adding a filter no longer force-flips the
Discover landing to the grid (only a header SEARCH does, since Discover has no
search rail) — so you stay on Discover with narrowed rails.

Frontend threads `filters` into api.plexDiscover/plexSurprise + their query keys
(plexSurprise is variadic now) and the watched-toggle setQueryData; PlexDiscover
reads them from usePlex. Tests: two discover filter-narrowing cases; PlexDiscover
dom test updated for the provider default + 3-arg call.

frontend/src/lib/queryKeys.ts

@@ -97,8 +97,9 @@ export const qk = {
   // both for a prefix invalidation after a watched-toggle. Surprise is its own key so the ↻ reshuffle
   // refetches it alone.
   plexDiscover: (...rest: unknown[]) => ["plex-discover", ...rest],
-  plexSurprise: (scope?: string) =>
-    scope !== undefined ? (["plex-surprise", scope] as const) : (["plex-surprise"] as const),
+  // Variadic like plexDiscover so it can key by scope + the active filters; `plexSurprise()` (no args)
+  // stays a prefix for the reshuffle/watched-toggle invalidations.
+  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
   // metadata is cached separately. Omitting it (e.g. for a prefix invalidation) still matches both.