fix: move migrations to correct directory

priv/repo/migrations/20260331000003_create_sensors.exs

@@ -0,0 +1,17 @@
+defmodule Localiser.Repo.Migrations.CreateSensors do
+  use Ecto.Migration
+
+  def change do
+    create table(:sensors) do
+      add :sensor_id, :string, null: false
+      add :room_id, references(:rooms, on_delete: :nilify_all)
+      add :x, :float
+      add :y, :float
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:sensors, [:sensor_id])
+    create index(:sensors, [:room_id])
+  end
+end