fix: move migrations to correct directory

priv/repo/migrations/20260416000001_add_is_admin_to_users.exs

@@ -0,0 +1,9 @@
+defmodule Localiser.Repo.Migrations.AddIsAdminToUsers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add :is_admin, :boolean, default: false, null: false
+    end
+  end
+end