feat(server): agent socket endpoint, clear online status on boot

server/lib/server_web/channels/agent_socket.ex

@@ -0,0 +1,12 @@
+defmodule ServerWeb.AgentSocket do
+  @moduledoc "Entry socket for agents. Actual authentication happens in HostChannel.join/3."
+  use Phoenix.Socket
+
+  channel "host:*", ServerWeb.HostChannel
+
+  @impl true
+  def connect(_params, socket, _connect_info), do: {:ok, socket}
+
+  @impl true
+  def id(_socket), do: nil
+end