Add backend ESLint configuration

backend/.eslintrc.js

@@ -0,0 +1,23 @@
+module.exports = {
+  env: {
+    browser: false,
+    es2021: true,
+    node: true,
+    jest: true
+  },
+  extends: [
+    'eslint:recommended'
+  ],
+  parserOptions: {
+    ecmaVersion: 'latest',
+    sourceType: 'module'
+  },
+  rules: {
+    'indent': ['error', 2],
+    'linebreak-style': ['error', 'unix'],
+    'quotes': ['error', 'single'],
+    'semi': ['error', 'always'],
+    'no-unused-vars': ['error', { 'argsIgnorePattern': '^_' }],
+    'no-console': ['warn', { allow: ['warn', 'error'] }]
+  }
+};