Web Push transport — delegates to a consumer-configured backend.

Configure a backend module in your app config and ash_dispatch will call its deliver/4 callback every time an event with a :push channel fires:

config :ash_dispatch, :push_backend, MyApp.Push

The backend module must implement AshDispatch.PushBackend:

defmodule MyApp.Push do
  @behaviour AshDispatch.PushBackend

  @impl true
  def deliver(receipt, context, channel, event_config) do
    # look up the user's subscriptions, encrypt per RFC 8291,
    # POST to each endpoint, prune the ones answering 404/410,
    # then mark the receipt :sent / :skipped / :failed.
  end
end

When no backend is configured the receipt is marked :skipped with error_message: "transport_not_implemented" — same shape as the SMS transport, so an app can declare :push channels before the backend exists without failing events.

Declare push channels as optional: true

Push is opt-in twice over: the browser must grant permission and the user must have visited on a device that supports it. A :push channel that is not optional: true turns "this user never allowed notifications" into a delivery failure. Mark it optional and let the recipient resolver soft-skip, the same way SMS soft-skips a recipient without a phone number.