feat: expose CRUD, onboarding, pubsub via web

lib/localiser/web/channels/particles_channel.ex

@@ -0,0 +1,21 @@
+defmodule Localiser.Web.Channels.ParticlesChannel do
+  use Phoenix.Channel
+
+  @impl true
+  def join("particles:" <> tag_id, _params, socket) do
+    Phoenix.PubSub.subscribe(Localiser.PubSub, "particles:#{tag_id}")
+    {:ok, assign(socket, :tag_id, tag_id)}
+  end
+
+  @impl true
+  def handle_info({:particles_updated, payload}, socket) do
+    push(socket, "particles_updated", %{
+      tag_id:    payload.tag_id,
+      estimate:  payload.estimate,
+      particles: payload.particles
+    })
+    {:noreply, socket}
+  end
+
+  def handle_info(_msg, socket), do: {:noreply, socket}
+end