Add logic to backfill scrobbled_tracks

Chokes on duplicate primary keys

lib/last_fm/worker/backfill_scrobbled_tracks.ex

@@ -0,0 +1,29 @@
+defmodule LastFm.Worker.BackfillScrobbledTracks do
+  use Oban.Worker, queue: :heavy_writes, max_attempts: 3
+
+  alias MusicLibrary.BackgroundRepo
+
+  @backfill_delay 5
+  @batch_size 100
+
+  @impl Oban.Worker
+  def perform(%{args: %{"to_uts" => to_uts}}) do
+    # importing is an all or nothing operation, which means that we can
+    # use the returning count to determine if we reached the end of the backfilling
+    # process.
+    case LastFm.Import.batch(to_uts: to_uts, limit: @batch_size) do
+      {@batch_size, nil} ->
+        next_to_uts = LastFm.lowest_scrobbled_at_uts()
+
+        %{"to_uts" => next_to_uts}
+        |> new(schedule_in: @backfill_delay)
+        |> BackgroundRepo.insert()
+
+      {other_count, nil} when is_integer(other_count) ->
+        :ok
+
+      error ->
+        error
+    end
+  end
+end