feat: expose CRUD, onboarding, pubsub via web

lib/localiser/domain/sensors.ex

@@ -9,6 +9,12 @@ defmodule Localiser.Domain.Sensors do
     Repo.all(Sensor)
   end
 
+  def list_unplaced do
+    Sensor |> where([s], is_nil(s.room_id)) |> Repo.all()
+  end
+
+  def any_placed?, do: Repo.exists?(from s in Sensor, where: not is_nil(s.room_id))
+
   def list_sensors_for_room(room_id) do
     Sensor
     |> where([s], s.room_id == ^room_id)