Add Artist#first_listed_on

Since an artist is considered to be listed when at least one of their
albums is published, I think it makes sense to say
`Artist#first_listed_on` should be the oldest of
`Album#first_published_on` on any of its albums. And that should be
correct even if an album has subsequently been unpublished.

I'm planning to use `Artist#first_listed_on` for the entry.published
property on an Atom feed for `ArtistsController#index`, i.e. for each
artist.

app/models/artist.rb

@@ -26,6 +26,10 @@ def listed?
     albums.where(publication_status: :published).any?
   end
 
+  def first_listed_on
+    albums.minimum(:first_published_on)
+  end
+
   def transcode_albums
     albums.each(&:transcode_tracks)
   end

test/models/artist_test.rb

@@ -40,6 +40,15 @@ class ArtistTest < ActiveSupport::TestCase
     assert artist.listed?
   end
 
+  test '#first_listed_on returns oldest Album#first_published_on' do
+    newer_album = build(:album, publication_status: :published, first_published_on: Date.parse('2023-01-02'))
+    older_album = build(:album, publication_status: :unpublished, first_published_on: Date.parse('2023-01-01'))
+    album_without_date = build(:album, publication_status: :pending, first_published_on: nil)
+    artist = create(:artist, albums: [newer_album, older_album, album_without_date])
+
+    assert_equal older_album.first_published_on, artist.first_listed_on
+  end
+
   test 'uses a friendly id' do
     artist = create(:artist, name: 'Rick Astley')