fix: actually include sensor firmware version string in repo, /version response

priv/repo/migrations/20260520192900_add_firmware_version_to_sensors.exs

@@ -0,0 +1,9 @@
+defmodule Localiser.Repo.Migrations.AddFirmwareVersionToSensors do
+  use Ecto.Migration
+
+  def change do
+    alter table(:sensors) do
+      add :firmware_version, :string
+    end
+  end
+end