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

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

apps/api/models/users/user_activity.mdl.js

@@ -0,0 +1,30 @@
+const { DataTypes } = require('sequelize');
+const sequelize = require('../../config/db.config');
+const mdl_Users = require('./users.mdl');
+
+const mdl_UserActivity = sequelize.define('UserActivity', {
+  activity_id: { type: DataTypes.BIGINT, primaryKey: true, autoIncrement: true },
+  user_id:     { type: DataTypes.BIGINT, allowNull: false, references: { model: mdl_Users, key: 'user_id' } },
+  session_id:  { type: DataTypes.BIGINT, allowNull: true },
+  action:      { type: DataTypes.STRING(100), allowNull: false },
+  entity_type: { type: DataTypes.STRING(50), allowNull: true },
+  entity_id:   { type: DataTypes.BIGINT, allowNull: true },
+  /**
+   * details JSONB — free-form context per action, e.g.:
+   *   login            : { session_id, reg_type }
+   *   deactivate_user  : { target_email }
+   *   lesson_read      : { lesson_uuid, status }
+   *   submit_task      : { task_id }
+   *   set_user_status  : { is_active }
+   */
+  details:     { type: DataTypes.JSONB, allowNull: true },
+  created_at:  { type: DataTypes.DATE, allowNull: true, defaultValue: DataTypes.NOW },
+}, {
+  tableName:  'user_activity',
+  timestamps: false,
+});
+
+mdl_UserActivity.belongsTo(mdl_Users, { foreignKey: 'user_id', as: 'user' });
+mdl_Users.hasMany(mdl_UserActivity, { foreignKey: 'user_id', as: 'activities' });
+
+module.exports = mdl_UserActivity;