Experimental Async Refreshes API (#34918)

app/controllers/concerns/async_refreshes_concern.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module AsyncRefreshesConcern
+  private
+
+  def add_async_refresh_header(async_refresh, retry_seconds: 3)
+    return unless async_refresh.running?
+
+    response.headers['Mastodon-Async-Refresh'] = "id=\"#{async_refresh.id}\", retry=#{retry_seconds}"
+  end
+end