feat: expose CRUD, onboarding, pubsub via web

lib/localiser/web/channels/room_channel.ex

@@ -0,0 +1,20 @@
+defmodule Localiser.Web.Channels.RoomChannel do
+  use Phoenix.Channel
+
+  @impl true
+  def join("rooms:occupancy", _params, socket) do
+    Phoenix.PubSub.subscribe(Localiser.PubSub, "rooms:occupancy")
+    {:ok, socket}
+  end
+
+  @impl true
+  def handle_info({:room_occupancy_changed, room_id, occupants}, socket) do
+    push(socket, "occupancy_changed", %{
+      room_id:   room_id,
+      occupants: MapSet.to_list(occupants)
+    })
+    {:noreply, socket}
+  end
+
+  def handle_info(_msg, socket), do: {:noreply, socket}
+end