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/i18n/locales/en/plex.json

@@ -253,5 +253,19 @@
     "movies": "Movies ({{count}})",
     "shows": "Shows ({{count}})",
     "empty": "No titles found for this person."
+  },
+  "view": {
+    "label": "View",
+    "browse": "Browse"
+  },
+  "discover": {
+    "tab": "Discover",
+    "continueWatching": "Continue watching",
+    "recentlyAdded": "Recently added",
+    "surprise": "Surprise me",
+    "reshuffle": "Reshuffle",
+    "byGenre": "Browse by genre",
+    "becauseWatched": "Because you watched {{title}}",
+    "empty": "Nothing to discover yet — run a Plex sync from the admin Config page."
   }
 }

frontend/src/i18n/locales/hu/plex.json

@@ -253,5 +253,19 @@
     "movies": "Filmek ({{count}})",
     "shows": "Sorozatok ({{count}})",
     "empty": "Nincs találat ehhez a személyhez."
+  },
+  "view": {
+    "label": "Nézet",
+    "browse": "Böngészés"
+  },
+  "discover": {
+    "tab": "Felfedezés",
+    "continueWatching": "Folytatás",
+    "recentlyAdded": "Nemrég hozzáadva",
+    "surprise": "Lepj meg",
+    "reshuffle": "Újrakeverés",
+    "byGenre": "Böngészés műfaj szerint",
+    "becauseWatched": "Mert megnézted: {{title}}",
+    "empty": "Még nincs mit felfedezni — futtass egy Plex-szinkront az admin Konfiguráció oldalról."
   }
 }