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.

backend/app/routes/plex/__init__.py

@@ -15,6 +15,7 @@ from app.routes.plex import (
     browse,
     collections,
     detail,
+    discover,
     people,
     playback,
     playlists,
@@ -25,6 +26,7 @@ router.include_router(admin.router)
 router.include_router(browse.router)
 router.include_router(collections.router)
 router.include_router(detail.router)
+router.include_router(discover.router)
 router.include_router(people.router)
 router.include_router(playback.router)
 router.include_router(playlists.router)