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.dom.test.tsx

@@ -77,6 +77,7 @@ const card = (id: string, title: string, type: "movie" | "show" = "movie") => ({
 const fullPayload = {
   scope: "both",
   continue_watching: [card("cw1", "Resume Me")],
+  recently_watched: [card("rw1", "Watched Me")],
   recently_added: [card("ra1", "Just Added")],
   genres: [{ value: "Drama", count: 12, art: [] }],
   because_you_watched: { anchor: { id: "a1", title: "Anchor" }, items: [card("bw1", "Rec One")] },
@@ -103,6 +104,7 @@ describe("PlexDiscover", () => {
       />,
     );
     await screen.findByText("plex.discover.continueWatching");
+    expect(screen.getByText("plex.discover.recentlyWatched")).toBeTruthy();
     expect(screen.getByText("plex.discover.recentlyAdded")).toBeTruthy();
     expect(screen.getByText("plex.discover.surprise")).toBeTruthy();
     expect(screen.getByText("plex.discover.byGenre")).toBeTruthy();
@@ -110,6 +112,7 @@ describe("PlexDiscover", () => {
     expect(screen.getByText("plex.discover.becauseWatched:Anchor")).toBeTruthy();
     // A card from each rail is present (title rendered in the poster label).
     expect(screen.getAllByText("Resume Me").length).toBeGreaterThan(0);
+    expect(screen.getAllByText("Watched Me").length).toBeGreaterThan(0);
     expect(screen.getAllByText("Just Added").length).toBeGreaterThan(0);
     expect(screen.getAllByText("Random Pick").length).toBeGreaterThan(0);
     expect(screen.getAllByText("Rec One").length).toBeGreaterThan(0);
@@ -215,6 +218,7 @@ describe("PlexDiscover", () => {
     plexDiscover.mockResolvedValue({
       scope: "both",
       continue_watching: [],
+      recently_watched: [],
       recently_added: [],
       genres: [],
       because_you_watched: null,

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")}