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.

frontend/src/lib/api/playlists.ts

@@ -68,8 +68,6 @@ export const playlistsApi = {
     req(`/api/playlists/${id}`, { method: "PATCH", body: JSON.stringify({ name }) }),
   deletePlaylist: (id: number, onYoutube = false) =>
     req(`/api/playlists/${id}${onYoutube ? "?on_youtube=true" : ""}`, { method: "DELETE" }),
-  // `position` (an index in the stored order) puts it back where a removal took it from — the
-  // list page's Undo; omitted, it is appended.
   addToPlaylist: (id: number, videoId: string) =>
     req(`/api/playlists/${id}/items`, {
       method: "POST",