Restore PushNotificationJob as shim for in-flight jobs during deploy

In-flight jobs queued before deploy would fail if the old class is
missing. This shim delegates to the new notification.push method.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/jobs/push_notification_job.rb

@@ -0,0 +1,7 @@
+class PushNotificationJob < ApplicationJob
+  discard_on ActiveJob::DeserializationError
+
+  def perform(notification)
+    notification.push
+  end
+end