Add Credo refactor checks and fix double Enum.map

Add 9 enabled + 1 disabled Credo checks from ex_slop README.
Fix MapMap finding in ListeningStats.update/1 (single pass).
Disable AppendSingleItem (10 pre-existing violations).

lib/music_library/listening_stats.ex

@@ -51,8 +51,7 @@ defmodule MusicLibrary.ListeningStats do
   def update(tracks) do
     track_params =
       tracks
-      |> Enum.map(fn t -> Map.take(t, @insertable_fields) end)
-      |> Enum.map(&Map.to_list/1)
+      |> Enum.map(fn t -> t |> Map.take(@insertable_fields) |> Map.to_list() end)
 
     {count, tracks} =
       Repo.insert_all(Track, track_params,