feat(plex): a Surprise reshuffle deals the previous draw last

The rail draws three unseen titles per kind under the sidebar filters; on a
small filtered pool every reshuffle dealt the same few again. The redraw now
sends the draw on screen (exclude=), the server deals the rest of the pool
first and tops up from the excluded titles only when the pool is short, so a
small pool still fills the rail. (The two discover.py baseline entries went
with the rewritten draw.)

frontend/src/components/PlexDiscover.dom.test.tsx

@@ -218,6 +218,9 @@ describe("PlexDiscover", () => {
     fireEvent.click(screen.getByRole("button", { name: "plex.discover.reshuffle" }));
     await waitFor(() => expect(plexSurprise).toHaveBeenCalledTimes(2));
     expect(plexDiscover).toHaveBeenCalledTimes(1); // the whole surface was NOT refetched
+    // The first draw asks for anything; the redraw hands over the one on screen to deal it last.
+    expect(plexSurprise.mock.calls[0]?.[4]).toEqual([]);
+    expect(plexSurprise.mock.calls[1]?.[4]).toEqual(["sp1"]);
   });
 
   it("shows the empty message when every rail is empty", async () => {