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

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

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

@@ -0,0 +1,21 @@
+const { DataTypes } = require("sequelize");
+const sequelize = require("../../config/db.config");
+
+const CourseInstructor = sequelize.define("CourseInstructor", {
+  id:           { type: DataTypes.BIGINT,      primaryKey: true, autoIncrement: true },
+  course_id:    { type: DataTypes.BIGINT,      allowNull: false },
+  user_id:      { type: DataTypes.BIGINT,      allowNull: true  },
+  display_name: { type: DataTypes.STRING(255), allowNull: false },
+  order_index:  { type: DataTypes.INTEGER,     allowNull: false, defaultValue: 0 },
+  created_by:   { type: DataTypes.BIGINT,      allowNull: true  },
+}, {
+  tableName:  "course_instructors",
+  timestamps: true,
+  createdAt:  "created_at",
+  updatedAt:  "updated_at",
+  indexes: [
+    { fields: ["course_id"] },
+  ],
+});
+
+module.exports = CourseInstructor;