feat(playlists): act on the selected rows from the toolbar, and undo either

While rows are selected the toolbar's second row is the band: the count,
the last target as one click, Move to, Remove, Clear selection. Both write
in one request and undo back to the places; a video the target held before
stays there. Delete takes the selection when there is one. A failure puts
the rows back ticked.

A row's own move goes through the same write, so it undoes too, and the
single-row move route and service function are gone. A playlist row drops
its grip below a 24rem table: the row menu keeps the moves.

backend/tests/test_reader_service.py

@@ -235,14 +235,13 @@ def test_a_move_never_leaves_a_video_in_two_lists(db, make_user):
     service.add_item(db, dst, "b", mark_dirty=False)
     db.commit()
     with pytest.raises(service.MoveRefused, match="same_playlist"):
-        service.move_item(db, src, src, "a")
-    service.move_item(db, src, dst, "a")
-    service.move_item(db, src, dst, "b")  # already in the target: dropped from the source
+        service.move_items(db, src, src, ["a"])
+    assert service.move_items(db, src, dst, ["a"]) == (["a"], [])
+    # already in the target: dropped from the source
+    assert service.move_items(db, src, dst, ["b"]) == ([], ["b"])
     db.commit()
     assert (_ids(db, src), _ids(db, dst)) == ([], ["b", "a"])
-    with pytest.raises(service.MoveRefused, match="not_in_playlist"):
-        service.move_item(db, src, dst, "a")
-    db.rollback()
+    assert service.move_items(db, src, dst, ["a"]) == ([], [])
 
 
 def test_a_reorder_keeps_what_it_left_out_after_the_listed(db, make_user):