Feature/#3

models/Friend.js

+// models/Friend.js
+
+const { DataTypes } = require('sequelize');
+const sequelize = require('../config/sequelize');
+const User = require('./User');
+
+const Friend = sequelize.define('Friend', {
+  type: {
+    type: DataTypes.ENUM('NORMAL', 'SPECIAL'),
+    allowNull: false,
+  },
+}, {
+  tableName: 'Friends',
+  timestamps: true,
+});
+
+Friend.belongsTo(User, { foreignKey: 'user_id', as: 'user' });
+Friend.belongsTo(User, { foreignKey: 'friend_id', as: 'friend' });
+
+User.hasMany(Friend, { foreignKey: 'user_id', as: 'friends' });
+User.hasMany(Friend, { foreignKey: 'friend_id', as: 'friendOf' });
+
+module.exports = Friend;