Add unique index to on MB id for artists

priv/repo/migrations/20240915130802_add_musicbrainz_id_index_to_artists.exs

@@ -0,0 +1,7 @@
+defmodule MusicLibrary.Repo.Migrations.AddMusicbrainzIdIndexToArtists do
+  use Ecto.Migration
+
+  def change do
+    create unique_index(:artists, [:musicbrainz_id])
+  end
+end