Use Ecto.UUID instead of hand-rolled type

test/music_library/scrobble_rules/scrobble_rule_test.exs

@@ -108,7 +108,7 @@ defmodule MusicLibrary.ScrobbleRules.ScrobbleRuleTest do
       changeset = ScrobbleRule.changeset(%ScrobbleRule{}, attrs)
       refute changeset.valid?
 
-      assert "must be a valid MusicBrainz ID (UUID format)" in errors_on(changeset).target_musicbrainz_id
+      assert "is invalid" in errors_on(changeset).target_musicbrainz_id
     end
 
     test "valid changeset with uppercase UUID" do

test/music_library_web/live/scrobble_rules_live_test.exs

@@ -162,7 +162,7 @@ defmodule MusicLibraryWeb.ScrobbleRulesLiveTest do
                  target_musicbrainz_id: "invalid-uuid"
                }
              )
-             |> render_change() =~ "must be a valid MusicBrainz ID"
+             |> render_change() =~ "is invalid"
     end
 
     test "updates form labels based on rule type", %{conn: conn} do