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

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

apps/api/models/tiers/plan_courses.mdl.js

@@ -0,0 +1,28 @@
+/***********************************************************************************************************************************************************************
+ * File Name: plan_courses.mdl.js
+ * Type of Program: Model
+ * Description: Junction table — links courses to a specific tier plan.
+ *              Composite UNIQUE on (plan_id, course_id) only prevents adding
+ *              the same course twice to the same plan — a course may belong
+ *              to many plans simultaneously (Tier Plans v2, silent duplication
+ *              across bundles is intentional; see admin/tiers.controller.js).
+ *              Bundling/display only, not an access-control mechanism — see
+ *              controllers/client/courses.controller.js (hasItemGrant) /
+ *              user_tier_grants for entitlement.
+ * Author: Kenneth Obsequio (@lash0000)
+ * Date Created: Jun. 6, 2026
+ ***********************************************************************************************************************************************************************/
+const { DataTypes } = require('sequelize');
+const sequelize = require('../../config/db.config');
+
+const mdl_PlanCourses = sequelize.define('PlanCourse', {
+  id:        { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true },
+  plan_id:   { type: DataTypes.BIGINT, allowNull: false },
+  course_id: { type: DataTypes.BIGINT, allowNull: false },
+}, {
+  tableName:  'plan_courses',
+  timestamps: true,
+  paranoid:   false,
+});
+
+module.exports = mdl_PlanCourses;