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

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

apps/api/utils/purchasable.util.js

@@ -0,0 +1,45 @@
+/***********************************************************************************************************************************************************************
+ * File Name: purchasable.util.js
+ * Type of Program: Utility
+ * Description: Resolves a Product's polymorphic target (purchasable_type +
+ *              purchasable_id → Course | Unit | Lesson) and the checkout
+ *              route for it. Products.mdl.js can't express a single
+ *              Sequelize association across three different target models,
+ *              so this is the shared lookup every consumer (access checks,
+ *              admin product CRUD, client checkout) goes through instead.
+ *
+ * Author: Kenneth Obsequio (@lash0000)
+ * Date Created: Aug. 1, 2026
+ ***********************************************************************************************************************************************************************/
+'use strict';
+
+const { Course } = require('../models/courses/courses.mdl');
+const Unit        = require('../models/courses/units.mdl');
+const Lesson       = require('../models/courses/lessons.mdl');
+
+const TYPE_MODELS = { course: Course, unit: Unit, lesson: Lesson };
+const TYPE_PK      = { course: 'course_id', unit: 'unit_id', lesson: 'lesson_id' };
+
+function getPurchasableModel(type) {
+  return TYPE_MODELS[type] ?? null;
+}
+
+// Returns the target row (needs at least uuid/course_id + subscription for
+// callers), or null if the type is unknown or the row doesn't exist.
+async function resolvePurchasable(type, id) {
+  const Model = TYPE_MODELS[type];
+  if (!Model) return null;
+  return Model.findOne({ where: { [TYPE_PK[type]]: id, deletedAt: null } });
+}
+
+// Course checkout stays keyed by numeric course_id (existing convention);
+// Unit/Lesson checkout is keyed by uuid, matching how /units/:uuid and
+// /lessons/:uuid already work everywhere else in the standalone consumption paths.
+function checkoutPath(type, record) {
+  if (type === 'course') return `/course/${record.course_id}/checkout`;
+  if (type === 'unit')   return `/units/${record.uuid}/checkout`;
+  if (type === 'lesson') return `/lessons/${record.uuid}/checkout`;
+  return null;
+}
+
+module.exports = { getPurchasableModel, resolvePurchasable, checkoutPath };