feat(plex): add a "Recently watched" rail to Discover (D)

A new Discover rail listing what the user last FULLY watched, newest first: a movie as its
own card, a show as the specific EPISODE watched (carrying its show title via leaf_card). Sits
above Recently added. Movies honour the sidebar filters like the other rails; episodes carry no
filterable metadata so they don't. Clicking a card plays it (the existing onCard path already
routes episodes to the player). Bilingual heading added.

Verified live: the rail renders watched movies + episodes with Watched badges.

Gate: tsc/lint/knip/prettier clean, 423 vitest; backend ruff clean, 13 discover tests (+ a
recently-watched movie+episode assertion).

frontend/src/components/PlexDiscover.tsx

@@ -143,6 +143,7 @@ export default function PlexDiscover({
   const bw = d.because_you_watched;
   const nothing =
     d.continue_watching.length === 0 &&
+    d.recently_watched.length === 0 &&
     d.recently_added.length === 0 &&
     d.genres.length === 0 &&
     surprise.length === 0 &&
@@ -165,6 +166,15 @@ export default function PlexDiscover({
         />
       )}
 
+      {d.recently_watched.length > 0 && (
+        <CardRail
+          title={t("plex.discover.recentlyWatched")}
+          ariaLabel={t("plex.discover.recentlyWatched")}
+          cards={d.recently_watched}
+          {...railProps}
+        />
+      )}
+
       {d.recently_added.length > 0 && (
         <CardRail
           title={t("plex.discover.recentlyAdded")}