E5 S4: Plex Discover rail-surface (default landing)

A dedicated Plex "Discover" landing — a stack of horizontal Carousel rails
served from ONE consolidated read (serve-from-DB, no live Plex/TMDB call):

- GET /api/plex/discover → continue_watching (in-progress movies + shows, most
  recently progressed) · recently_added · genres (top-N tiles) ·
  because_you_watched (recs of the most-recent watched, TMDB-enriched movie,
  resolved to library rows by guid via serve_block's resolver).
- GET /api/plex/surprise → a fresh random, scope-bounded sample for the ↻ rail.

Frontend: PlexDiscover.tsx (rails reuse the S1 Carousel + S3 PlexPosterCard;
colored genre tiles → tap to filter the grid; ↻ reshuffles the surprise rail
alone). Discover is now the root sub-view; the flat grid becomes the bookmarkable
"Browse all" secondary view (?v=browse) reached by a header search/filter (a
browse-intent flip) or the new Discover⟷Browse toggle.

resolve_recs is promoted from _resolve_recs (now cross-module). Tests:
test_plex_discover.py (rail shape/order, continue-watching, because-you-watched
+ null, surprise bounds, scope) and PlexDiscover.dom.test.tsx; plexSubUrl
round-trip updated for discover-root/grid-browse. Backend lane 393, vitest 332.

frontend/src/lib/api/plex.ts

@@ -263,6 +263,17 @@ export interface PlexPlaySession {
   start_s: number;
 }
 
+// Discover surface (E5 S4): the consolidated rail payload. Every rail is a plain PlexCard list the
+// dedicated Discover landing renders as a horizontal Carousel; `because_you_watched` is null when the
+// user has no TMDB-enriched watch history (rail hidden). Served from DB — no live Plex/TMDB call.
+export interface PlexDiscover {
+  scope: string;
+  continue_watching: PlexCard[];
+  recently_added: PlexCard[];
+  genres: { value: string; count: number }[]; // top genres → tap a tile to filter the grid
+  because_you_watched: { anchor: { id: string; title: string }; items: PlexCard[] } | null;
+}
+
 // Two-way Plex watch-state sync link status (owner account; Phase A).
 export interface PlexWatchLink {
   linked: boolean;
@@ -411,6 +422,18 @@ export const plexApi = {
       method: "PUT",
       body: JSON.stringify({ item_rating_keys: itemRks }),
     }),
+  // Discover landing rails in one request (serve-from-DB). `lang` picks the genre-tile vocabulary.
+  plexDiscover: (scope: string, lang?: string): Promise<PlexDiscover> => {
+    const u = new URLSearchParams({ scope });
+    if (lang === "hu") u.set("lang", "hu");
+    return req(`/api/plex/discover?${u.toString()}`);
+  },
+  // A fresh random sample for the Surprise-me rail — a new draw every call (the ↻ refetch).
+  plexSurprise: (scope: string, limit?: number): Promise<{ scope: string; items: PlexCard[] }> => {
+    const u = new URLSearchParams({ scope });
+    if (limit) u.set("limit", String(limit));
+    return req(`/api/plex/surprise?${u.toString()}`);
+  },
   plexShow: (id: string, lang?: string): Promise<PlexShowDetail> =>
     req(`/api/plex/show/${encodeURIComponent(id)}${lang === "hu" ? "?lang=hu" : ""}`),
   // Mark a whole show / season watched or unwatched (all its episodes) for the current user.