fix(playlists): overlapping writes, and what the second review found

A read from before a write no longer brings a row back: a write calls off the
read on its way, what lands while one is out stays off the screen, and the last
write out does the refresh. A failure puts back its own rows, an accepted reorder
brings back none, and an Undo reads past the cache. A list the cache has dropped
is read before a removal, so its Undo has places.

undo-move refuses the same list twice, positions cannot repeat, and undo-move
and the Plex key bodies refuse a body without their videos. Focus goes to the
header's box when the last shown row or Save this order goes.

backend/app/reader/service.py

@@ -559,7 +559,9 @@ def undo_move(
     `dst`, which held them before. A moved video `dst` no longer holds has gone on from there, and
     bringing it back would leave it in lists the reader never put it in, so it stays out, as a
     move skips a video a concurrent edit took out. `positions` pairs with `moved` + `already_there`.
-    Says how many came back."""
+    Says how many came back. Raises `MoveRefused` for the same list twice, as a move does."""
+    if src.id == dst.id:
+        raise MoveRefused("same_playlist")
     _lock_both(db, src, dst)
     wanted = set(moved)
     rows = {it.video_id: it for it in queries.items(db, dst.id) if it.video_id in wanted}