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/components/plexSubUrl.test.ts

@@ -5,6 +5,7 @@ import { PLEX_SUB_URL as U, type Sub } from "./plexSubUrl";
 
 describe("PLEX_SUB_URL encode", () => {
   it("mirrors each bookmarkable sub-view to ?v=<kind>:<args>", () => {
+    expect(U.encode({ kind: "grid" })).toBe("browse"); // the flat "Browse all" grid (E5 S4)
     expect(U.encode({ kind: "show", id: "s1" })).toBe("show:s1");
     expect(U.encode({ kind: "season", showId: "s1", seasonId: "se2" })).toBe("season:s1:se2");
     expect(U.encode({ kind: "info", id: "m9" })).toBe("info:m9");
@@ -12,14 +13,15 @@ describe("PLEX_SUB_URL encode", () => {
     expect(U.encode({ kind: "playlist", id: 12 })).toBe("playlist:12");
   });
 
-  it("does NOT mirror the root grid or the ephemeral player (encode → null)", () => {
-    expect(U.encode({ kind: "grid" })).toBeNull();
+  it("does NOT mirror the root Discover landing or the ephemeral player (encode → null)", () => {
+    expect(U.encode({ kind: "discover" })).toBeNull();
     expect(U.encode({ kind: "player", id: "m9" })).toBeNull();
   });
 });
 
 describe("PLEX_SUB_URL round-trip", () => {
   const cases: Sub[] = [
+    { kind: "grid" },
     { kind: "show", id: "s1" },
     { kind: "season", showId: "s1", seasonId: "se2" },
     { kind: "info", id: "m9" },