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

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

apps/api/models/courses/lesson_objective.mdl.js

@@ -0,0 +1,15 @@
+const { DataTypes } = require("sequelize");
+const sequelize = require("../../config/db.config");
+
+const LessonObjective = sequelize.define("LessonObjective", {
+  objective_id: { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true },
+  lesson_id:    { type: DataTypes.BIGINT, allowNull: false },
+  text:         { type: DataTypes.TEXT, allowNull: false },
+  order_index:  { type: DataTypes.INTEGER, defaultValue: 0 },
+}, {
+  tableName: "lesson_objectives",
+  timestamps: true,
+  paranoid: true,
+});
+
+module.exports = LessonObjective