Simplify data model

Artists can be embedded inside records - given the projected size of the
dataset (a few 1000s in the worst case) relationships can be built in
memory without the need to be denormalized into separate tables.

priv/repo/migrations/20240915181757_embed_artists.exs

@@ -0,0 +1,12 @@
+defmodule MusicLibrary.Repo.Migrations.EmbedArtists do
+  use Ecto.Migration
+
+  def change do
+    drop table(:artists_records)
+    drop table(:artists)
+
+    alter table(:records) do
+      add :artists, :map
+    end
+  end
+end