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

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

apps/api/database/migrations/20270101000070-create-course-instructors.js

@@ -0,0 +1,37 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.createTable('course_instructors', {
+      id:           { type: Sequelize.BIGINT, primaryKey: true, autoIncrement: true },
+      course_id:    { type: Sequelize.BIGINT, allowNull: false, references: { model: 'courses', key: 'course_id' }, onDelete: 'CASCADE' },
+      user_id:      { type: Sequelize.BIGINT, allowNull: true, references: { model: 'users', key: 'user_id' }, onDelete: 'CASCADE' },
+      order_index:  { type: Sequelize.BIGINT, allowNull: false, defaultValue: 0 },
+      display_name: { type: Sequelize.STRING(255), allowNull: false, defaultValue: '' },
+      created_by:   { type: Sequelize.BIGINT, allowNull: true },
+      created_at:   { type: Sequelize.DATE, allowNull: false, defaultValue: Sequelize.NOW },
+      updated_at:   { type: Sequelize.DATE, allowNull: false, defaultValue: Sequelize.NOW },
+    });
+
+    await queryInterface.addConstraint('course_instructors', {
+      fields: ['course_id', 'user_id'],
+      type: 'unique',
+      name: 'course_instructors_course_id_user_id_key',
+    });
+
+    await queryInterface.addIndex('course_instructors', ['course_id'], { name: 'idx_course_instructors_course' });
+
+    // Partial unique index — same columns as the constraint above, scoped to
+    // non-null user_id. Raw SQL since queryInterface addIndex's `where`
+    // support is unreliable for IS NOT NULL across dialects.
+    await queryInterface.sequelize.query(`
+      CREATE UNIQUE INDEX idx_course_instructors_user
+      ON course_instructors (course_id, user_id)
+      WHERE user_id IS NOT NULL;
+    `);
+  },
+
+  async down(queryInterface) {
+    await queryInterface.dropTable('course_instructors');
+  },
+};