fix: move migrations to correct directory

priv/repo/migrations/20260331000006_create_users.exs

@@ -0,0 +1,14 @@
+defmodule Localiser.Repo.Migrations.CreateUsers do
+  use Ecto.Migration
+
+  def change do
+    create table(:users) do
+      add :username, :string, null: false
+      add :password_hash, :string, null: false
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:users, [:username])
+  end
+end