chore: relocate backend into apps/api ahead of monorepo merge

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/api/database/migrations/20270101000077-add-lessons-subscription.js

@@ -0,0 +1,18 @@
+'use strict';
+
+// Standalone tier gate for Lessons, mirroring units.subscription (see
+// 20270101000012-create-units.js) — null means open (or gated only via an
+// attached unit/course). Full parity with the Unit-level gate added for the
+// standalone Units/Lessons tier-gating extension.
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.addColumn('lessons', 'subscription', {
+      type: Sequelize.STRING(50),
+      allowNull: true,
+    });
+  },
+
+  async down(queryInterface) {
+    await queryInterface.removeColumn('lessons', 'subscription');
+  },
+};