Add the SyncWorker and its cron configuration for Oban.

lib/decentralised_book_index/sync/sync_workers/sync_worker.ex

@@ -0,0 +1,13 @@
+defmodule DecentralisedBookIndex.SyncWorker do
+  use Oban.Worker,
+    queue: :default,
+    max_attempts: 2,
+    tags: ["sync", "cron"]
+
+  require Logger
+
+  @impl Oban.Worker
+  def perform(%Oban.Job{args: args} = job) do
+    SyncServerTask.sync_all()
+  end
+end