Migrate codebase from CommonJS to ES Modules

- Convert all require()/module.exports to import/export across 260+ files
- Add "type": "module" to package.json to enable ESM by default
- Add migrations/package.json with "type": "commonjs" to keep db-migrate compatible
- Convert eslint.config.js to ESM with sourceType: "module"
- Replace __dirname/__filename with import.meta.dirname/import.meta.filename
- Replace require.main === module with process.argv[1] === import.meta.filename
- Remove 'use strict' directives (implicit in ESM)
- Convert dynamic require() in switch statements to static import lookup maps
  (dns.js, domains.js, backupformats.js, backupsites.js, network.js)
- Extract self-referencing exports.CONSTANT patterns into standalone const
  declarations (apps.js, services.js, locks.js, users.js, mail.js, etc.)
- Lazify SERVICES object in services.js to avoid circular dependency TDZ issues
- Add clearMailQueue() to mailer.js for ESM-safe queue clearing in tests
- Add _setMockApp() to ldapserver.js for ESM-safe test mocking
- Add _setMockResolve() wrapper to dig.js for ESM-safe DNS mocking in tests
- Convert backupupload.js to use dynamic imports so --check exits before
  loading the module graph (which requires BOX_ENV)
- Update check-install to use ESM import for infra_version.js
- Convert scripts/ (hotfix, release, remote_hotfix.js, find-unused-translations)
- All 1315 tests passing

Migration stats (AI-assisted using Cursor with Claude):
- Wall clock time: ~3-4 hours
- Assistant completions: ~80-100
- Estimated token usage: ~1-2M tokens

Co-authored-by: Cursor <cursoragent@cursor.com>

src/test/syncer-test.js

@@ -1,20 +1,20 @@
 /* global it:false */
+
+import * as common from './common.js';
+const { createTree } = common;
+import DataLayout from '../datalayout.js';
+import { execSync } from 'node:child_process';
+import expect from 'expect.js';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import paths from '../paths.js';
+import safe from 'safetydance';
+import * as syncer from '../syncer.js';
+
 /* global describe:false */
 /* global before:false */
 
-'use strict';
-
-const createTree = require('./common.js').createTree,
-    DataLayout = require('../datalayout.js'),
-    execSync = require('node:child_process').execSync,
-    expect = require('expect.js'),
-    fs = require('node:fs'),
-    os = require('node:os'),
-    path = require('node:path'),
-    paths = require('../paths.js'),
-    safe = require('safetydance'),
-    syncer = require('../syncer.js');
-
 const gTmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'syncer-test')),
     gCacheFile = path.join(paths.BACKUP_INFO_DIR, 'syncer-test.sync.cache');