Fix scrobble tracks schema

Remove the primary key, and just set a unique index on scrobbled time
and title, which allows skipping duplicates.

lib/last_fm/import.ex

@@ -20,7 +20,8 @@ defmodule LastFm.Import do
       # HACK: if two tracks happen to have the exact same scrobbled_at_uts,
       # we move it by a sec.
       MusicLibrary.Repo.insert_all(LastFm.Track, track_params,
-        on_conflict: [inc: [scrobbled_at_uts: -1]]
+        on_conflict: :nothing,
+        conflict_target: [:scrobbled_at_uts, :title]
       )
     end
   end

priv/repo/migrations/20250530203701_create_scrobbled_tracks.exs

@@ -3,7 +3,7 @@ defmodule MusicLibrary.Repo.Migrations.CreateScrobbledTracks do
 
   def change do
     create table(:scrobbled_tracks, primary_key: false) do
-      add :scrobbled_at_uts, :integer, primary_key: true
+      add :scrobbled_at_uts, :integer
       add :musicbrainz_id, :string
       add :title, :string
       add :cover_url, :string
@@ -12,5 +12,7 @@ defmodule MusicLibrary.Repo.Migrations.CreateScrobbledTracks do
       add :album, :map
       add :last_fm_data, :map
     end
+
+    create index(:scrobbled_tracks, [:scrobbled_at_uts, :title], unique: true)
   end
 end