Consolidate push jobs into single Notification::PushJob

Replace separate WebPushJob and NativePushJob with a single PushJob
that calls notification.push, which iterates over registered targets.
Each target handles its own delivery - Web pushes synchronously via
the pool, Native enqueues device-level jobs via deliver_later_to.

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

saas/test/models/notification/push_target/native_test.rb

@@ -199,9 +199,4 @@ class Notification::PushTarget::NativeTest < ActiveSupport::TestCase
     assert_equal @notification.creator.name, native.data[:creator_name]
   end
 
-  test "push_later enqueues Notification::NativePushJob" do
-    assert_enqueued_with(job: Notification::NativePushJob, args: [ @notification ]) do
-      Notification::PushTarget::Native.push_later(@notification)
-    end
-  end
 end