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

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

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

@@ -0,0 +1,15 @@
+const { DataTypes } = require("sequelize");
+const sequelize = require("../../config/db.config");
+
+const CoursePrerequisite = sequelize.define("CoursePrerequisite", {
+  prereq_id:  { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true },
+  course_id:  { type: DataTypes.BIGINT, allowNull: false },  // the course that HAS this prereq
+  ref_type:   { type: DataTypes.ENUM("course", "unit", "lesson"), allowNull: false },
+  ref_id:     { type: DataTypes.BIGINT, allowNull: false },   // FK to courses/units/lessons
+  order_index:{ type: DataTypes.INTEGER, defaultValue: 0 },
+}, {
+  tableName: "course_prerequisites",
+  timestamps: true,
+});
+
+module.exports = CoursePrerequisite