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

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

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

@@ -0,0 +1,30 @@
+const { DataTypes } = require("sequelize");
+const sequelize = require("../../config/db.config");
+
+// Standalone entity — no unit_id / order_index here. A Lesson is attached to
+// zero or more Units through unit_lessons, where per-unit ordering lives.
+const Lesson = sequelize.define("Lesson", {
+  lesson_id: { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true, hidden: true, order: 0, filterable: false },
+  uuid: { type: DataTypes.UUID, defaultValue: DataTypes.UUIDV4, allowNull: false, unique: true, hidden: true, order: 0, filterable: false },
+
+  title: { type: DataTypes.STRING(255), allowNull: false, label: "Title", hidden: false, order: 1, filterable: true },
+  // order 2 is reserved for the computed "Affiliated" (course_count) column, order 3 for computed "Course Status" — see LESSON_LIST_COMPUTED in lessons.controller.js
+  subscription: { type: DataTypes.STRING(50), allowNull: true, label: "Subscription", hidden: false, order: 4, filterable: true, comment: "Optional direct tier gate for standalone lessons — null means open (or gated only via an attached unit)." },
+  description: { type: DataTypes.TEXT, allowNull: true, label: "Description", hidden: true, order: 0, filterable: false },
+
+  duration_seconds: { type: DataTypes.INTEGER, allowNull: true, defaultValue: 0, filterable: false }, // computed from blocks on save
+
+  createdBy: { type: DataTypes.BIGINT, allowNull: true, label: "Created By", filterable: true },
+  updatedBy: { type: DataTypes.BIGINT, allowNull: true, label: "Updated By", filterable: true },
+  deletedBy: { type: DataTypes.BIGINT, allowNull: true, label: "Deleted By", filterable: true },
+}, {
+  tableName: "lessons",
+  timestamps: true,
+  paranoid: true,
+  indexes: [
+    { fields: ["uuid"] },
+    { fields: ["deletedAt"] },
+  ],
+});
+
+module.exports = Lesson;