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/backupformats.js

@@ -1,20 +1,19 @@
-'use strict';
+import assert from 'node:assert';
+import BoxError from './boxerror.js';
+import * as tgzFormat from './backupformat/tgz.js';
+import * as rsyncFormat from './backupformat/rsync.js';
 
-exports = module.exports = {
+export {
     api,
     validateFormat,
 };
 
-const assert = require('node:assert'),
-    BoxError = require('./boxerror.js');
+const FORMATS = { tgz: tgzFormat, rsync: rsyncFormat };
 
 function api(format) {
     assert.strictEqual(typeof format, 'string');
 
-    switch (format) {
-    case 'tgz': return require('./backupformat/tgz.js');
-    case 'rsync': return require('./backupformat/rsync.js');
-    }
+    if (FORMATS[format]) return FORMATS[format];
 
     throw new BoxError(BoxError.INTERNAL_ERROR, `Undefined format ${format}`);
 }