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

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

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

@@ -0,0 +1,38 @@
+'use strict';
+const { DataTypes } = require('sequelize');
+const sequelize     = require('../../config/db.config');
+const { Course }    = require('./courses.mdl');
+
+// Polymorphic target — a product is sold against a Course (purchasable_type +
+// purchasable_id). Unit/Lesson individual purchase was removed; the schema
+// stays polymorphic (purchasable_type/purchasable_id, not a course_id FK)
+// since existing rows and course_purchases still key off it. See
+// utils/purchasable.util.js for the type -> model/checkout-path resolver used
+// by every consumer (access checks, admin CRUD, checkout).
+//
+// constraints: false below because purchasable_id doesn't point at a single
+// table — referential integrity across the three possible targets is
+// enforced at the application layer only (see
+// 20270101000078-generalize-products-purchasable.js).
+const mdl_Product = sequelize.define('Product', {
+  id:               { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true },
+  purchasable_type: { type: DataTypes.STRING(10), allowNull: false, defaultValue: 'course' },
+  purchasable_id:   { type: DataTypes.BIGINT, allowNull: false },
+  name:        { type: DataTypes.STRING(200), allowNull: false },
+  description: { type: DataTypes.TEXT, allowNull: true },
+  price:       { type: DataTypes.DECIMAL(10, 2), allowNull: false },
+  currency:    { type: DataTypes.STRING(10), allowNull: false, defaultValue: 'USD' },
+  access_days: { type: DataTypes.INTEGER, allowNull: true },
+  is_active:   { type: DataTypes.BOOLEAN, allowNull: false, defaultValue: true },
+}, {
+  tableName:  'products',
+  timestamps: true,
+  paranoid:   true,
+});
+
+// Scoped hasOne — Sequelize automatically adds the matching purchasable_type
+// filter to the join, so existing `include: [{ model: mdl_Product, as:
+// 'product' }]` call sites on Course keep working unchanged.
+Course.hasOne(mdl_Product,  { foreignKey: 'purchasable_id', constraints: false, scope: { purchasable_type: 'course' }, as: 'product' });
+
+module.exports = mdl_Product;