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.ts

@@ -5,6 +5,7 @@ import { parsePositiveIntId } from "../lib/parseId";
 // encode/decode round-trip (and its guards) can be unit-tested without pulling the whole component
 // (and its lazy hls.js player) into the test graph.
 export type Sub =
+  | { kind: "discover" }
   | { kind: "grid" }
   | { kind: "show"; id: string }
   | { kind: "season"; showId: string; seasonId: string }
@@ -14,12 +15,16 @@ export type Sub =
   | { kind: "playlist"; id: number };
 
 // Mirror the Plex sub-view to ?v=<kind>:<args> so a show / season / movie-info / person / playlist page
-// is bookmarkable. Grid (root) and the ephemeral player are not mirrored (encode → null). The person
-// name is the whole tail after the first ":", so a name containing ":" round-trips.
+// — and the flat browse grid — are bookmarkable. Discover is the ROOT landing (E5 S4), so it (and the
+// ephemeral player) is not mirrored (encode → null); the grid gets `?v=browse` so "Browse all" is a
+// distinct, shareable URL. The person name is the whole tail after the first ":", so a name containing
+// ":" round-trips.
 export const PLEX_SUB_URL: SubviewUrl<Sub> = {
   key: "v",
   encode: (v) => {
     switch (v.kind) {
+      case "grid":
+        return "browse";
       case "show":
         return `show:${v.id}`;
       case "season":
@@ -31,7 +36,7 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
       case "playlist":
         return `playlist:${v.id}`;
       default:
-        return null; // grid + the ephemeral player are not URL-mirrored
+        return null; // discover (root) + the ephemeral player are not URL-mirrored
     }
   },
   decode: (raw) => {
@@ -39,6 +44,8 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
     const kind = i === -1 ? raw : raw.slice(0, i);
     const arg = i === -1 ? "" : raw.slice(i + 1);
     switch (kind) {
+      case "browse":
+        return { kind: "grid" };
       case "show":
         return arg ? { kind: "show", id: arg } : null;
       case "info":