Fix CI: migrate ESLint to v9 with neostandard flat config

[dependabot]

npm ci was failing because Dependabot bumped eslint to ^10.4.0, but eslint-plugin-import@2.32.0 only peers up to ESLint 9, and the legacy eslint-config-standard@14 + eslint-plugin-node@11 stack uses APIs removed in ESLint 9+ (context.getScope()).

Changes

• ESLint version: ^8.57.1 → ^9.39.4 (highest version compatible with current plugin peer ranges)
• Config migration: .eslintrc.json → eslint.config.js (flat config, required from ESLint 9)
• Plugin stack replaced: eslint-config-standard@14, eslint-plugin-node@11 (deprecated), eslint-plugin-standard@4 (deprecated), eslint-plugin-import, eslint-plugin-promise all removed in favour of neostandard — the official ESLint-9 successor maintained by the standard team

New eslint.config.js:

"use strict";

const neostandard = require("neostandard");

module.exports = neostandard({
  globals: { test: true, expect: true },
  noStyle: true,
});

Preserves the same lint behaviour: Standard rules, Jest globals, all style rules disabled.