Upgrade to ESLint 9 with flat config format

[Copilot]

Migrates from ESLint 8 to ESLint 9, adopting the new flat config format required by the v9 migration path.

Changes

Core upgrades:

• eslint: 8.46.0 → 9.39.2
• eslint-plugin-jest: 27.2.3 → 28.14.0
• eslint-plugin-import: 2.30.0 → 2.32.0
• Added: @eslint/js, typescript-eslint, globals

Configuration:

• Migrated .eslintrc.js → eslint.config.js (flat config format)
• Removed eslint-config-standard and related plugins (no ESLint 9 support)
• Maintained all existing rules, TypeScript configs, and Jest test file patterns
• Moved reportUnusedDisableDirectives to linterOptions

Dependabot:

• Removed eslint* from ignore list

Config structure

// Old: .eslintrc.js (extends, plugins as strings)
module.exports = {
  extends: ['standard', 'prettier'],
  plugins: ['import', 'jest'],
  overrides: [{ files: ['*.ts'], parser: '@typescript-eslint/parser' }]
}

// New: eslint.config.js (flat array of configs)
module.exports = [
  { files: ['**/*.js'], plugins: { import, jest }, rules: {...} },
  ...tseslint.config(...tseslint.configs.recommended),
  { files: ['**/*.ts'], plugins: { '@typescript-eslint': tseslint.plugin } }
]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Upgrade to eslint9</issue_title>
<issue_description>### What

It's a bigger change with the new config file https://eslint.org/docs/latest/use/migrate-to-9.0.0 so worth its own ticket to plan work on.

[!NOTE]
Please remember to remove the relevant packages from Dependabot's ignore list once upgraded.

Definition of Done

• Has been deployed to production
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Upgrade to eslint9 #311

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