chore: add migrations to git

migrations/20260331000004_create_sensor_calibrations.exs

@@ -0,0 +1,15 @@
+defmodule Localiser.Repo.Migrations.CreateSensorCalibrations do
+  use Ecto.Migration
+
+  def change do
+    create table(:sensor_calibrations) do
+      add :sensor_id, references(:sensors, on_delete: :delete_all), null: false
+      add :rssi_ref, :integer, null: false
+      add :path_loss_exp, :float, null: false
+      add :calibrated_at, :utc_datetime, null: false
+      add :inserted_at, :utc_datetime, null: false
+    end
+
+    create index(:sensor_calibrations, [:sensor_id])
+  end
+end