test: close the last coverage debt the roadmap still listed

- `overflow-clip` gets a safelist anchor in index.css (its only occurrence is a `.ts` constant),
  and the chip-lane contract keeps it there.
- PlexPlayer's Backspace ownership rule is pinned on all three branches: nothing above, a foreign
  layer above (it defers), and its own panel open under a layer (it still closes the panel).
- Removing a playlist item bumps the list's `updated_at`, like the three other item-only writes.
- Seen in a real browser on localdev: an exhausted comfort-shelf tile (an empty pinned playlist) is
  focusable, aria-disabled, and its Shift+F10 menu offers Play off and Remove from shelf.

backend/tests/test_playlist_move.py

@@ -235,6 +235,13 @@ def test_item_edits_bump_updated_at(db, make_user, as_user):
     assert db.get(Playlist, src.id).updated_at > old
     assert db.get(Playlist, dst.id).updated_at > old
 
+    # …and removing one, the fourth item-only write (it had no test: E10 review coverage debt).
+    src.updated_at = old
+    db.commit()
+    assert client.delete(f"/api/playlists/{src.id}/items/v1").status_code == 200
+    db.expire_all()
+    assert db.get(Playlist, src.id).updated_at > old
+
 
 def test_the_list_endpoint_publishes_updated_at(db, make_user, as_user):
     user = make_user()