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

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

apps/api/database/migrations/20270101000074-create-course-achievements.js

@@ -0,0 +1,25 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.createTable('course_achievements', {
+      id:              { type: Sequelize.BIGINT, primaryKey: true, autoIncrement: true },
+      course_id:       { type: Sequelize.BIGINT, allowNull: false },
+      achievement_key: { type: Sequelize.STRING(100), allowNull: false },
+      order_index:     { type: Sequelize.INTEGER, allowNull: false, defaultValue: 0 },
+      createdAt:       { type: Sequelize.DATE, allowNull: false },
+      updatedAt:       { type: Sequelize.DATE, allowNull: false },
+    });
+
+    await queryInterface.addIndex('course_achievements', ['course_id']);
+    await queryInterface.addConstraint('course_achievements', {
+      fields: ['course_id', 'achievement_key'],
+      type: 'unique',
+      name: 'course_achievements_course_id_key_unique',
+    });
+  },
+
+  async down(queryInterface) {
+    await queryInterface.dropTable('course_achievements');
+  },
+};