fix(plex): Restart only where an episode has a resume point; the Surprise draw is re-read, not redrawn

Show cards carry resumable (ShowState, from the same rollup) so a part-watched
show whose next episode is untouched offers Play, not a Continue and a Restart
that do the same thing; plexStarted is the one rule for cards and menus. The
Surprise rail's key sits under the discover root: whatever refreshes the rails
re-reads the draw (keep=) with its watch state now, kept past the 5-minute gc,
and only the reshuffle deals. Closing the player refreshes the Discover rails.
A show card's play acts on the last click only, and only where it was made.

backend/tests/test_plex_discover.py

@@ -331,6 +331,20 @@ def test_a_reshuffle_deals_other_titles_first(db, monkeypatch, override_dep):
     assert set(movies("/api/plex/surprise?scope=movie&exclude=m2&exclude=m3")) == {"m2", "m3"}
 
 
+def test_a_kept_draw_is_read_again_not_redrawn(db, monkeypatch, override_dep):
+    # The rail's draw stands until the ↻, but its cards' watch state moves: `keep` answers the titles on
+    # screen, in their order and as they stand now, a watched one included; the scope still bounds it.
+    user = _seed(db)
+    url = "/api/plex/surprise?scope=both&keep=m3&keep=sh1&keep=m1&keep=nope"
+    items = _get(db, user, monkeypatch, override_dep, url).json()["items"]
+    assert _ids(items) == ["m3", "sh1", "m1"]
+    by_id = {c["id"]: c for c in items}
+    assert by_id["m1"]["status"] == "watched"
+    assert by_id["m3"]["position_seconds"] > 0
+    movie_only = "/api/plex/surprise?scope=movie&keep=sh1&keep=m2"
+    assert _ids(_get(db, user, monkeypatch, override_dep, movie_only).json()["items"]) == ["m2"]
+
+
 def test_discover_scope_movie_excludes_shows(db, monkeypatch, override_dep):
     user = _seed(db)
     d = _get(db, user, monkeypatch, override_dep, "/api/plex/discover?scope=movie").json()