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

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

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

@@ -0,0 +1,16 @@
+const { DataTypes } = require("sequelize");
+const sequelize     = require("../../config/db.config");
+
+const LessonPage = sequelize.define("LessonPage", {
+  page_id:   { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true },
+  lesson_id: { type: DataTypes.BIGINT, allowNull: false, unique: true },
+  blocks:    { type: DataTypes.JSONB,  allowNull: false, defaultValue: [] },
+  createdBy: { type: DataTypes.BIGINT, allowNull: true, label: "Created By" },
+  updatedBy: { type: DataTypes.BIGINT, allowNull: true, label: "Updated By" },
+}, {
+  tableName:  "lesson_pages",
+  timestamps: true,
+  paranoid:   false,
+});
+
+module.exports = LessonPage;