refactor(esm): migrate server and tests from CommonJS to ESM

Convert all server/, test, and setup files from require()/module.exports
to import/export syntax. Activate ESM globally via "type": "module" in
package.json and load dotenv via --import dotenv/config in npm scripts.

server/services/apple-calendar.js

@@ -12,12 +12,10 @@
  *   apple_last_sync - ISO-8601-Timestamp des letzten Syncs
  */
 
-'use strict';
-
-const { createLogger } = require('../logger');
+import { createLogger } from '../logger.js';
 const log = createLogger('Apple');
 
-const db = require('../db');
+import * as db from '../db.js';
 
 const APPLE_COLOR = '#FC3C44';
 
@@ -407,4 +405,4 @@ async function sync() {
   log.info(`Sync abgeschlossen - ${totalObjects} Objekte aus ${syncCalendars.length} Kalendern inbound, ${localEvents.length} lokal → iCloud.`);
 }
 
-module.exports = { sync, getStatus, saveCredentials, clearCredentials, testConnection };
+export { sync, getStatus, saveCredentials, clearCredentials, testConnection };

server/services/google-calendar.js

@@ -11,14 +11,12 @@
  *   google_last_sync      - ISO-8601-Timestamp des letzten erfolgreichen Syncs
  */
 
-'use strict';
-
-const { createLogger } = require('../logger');
+import { createLogger } from '../logger.js';
 const log = createLogger('Google');
 
-const { google } = require('googleapis');
-const crypto = require('crypto');
-const db = require('../db');
+import { google } from 'googleapis';
+import crypto from 'crypto';
+import * as db from '../db.js';
 
 const GOOGLE_COLOR = '#4285F4';
 
@@ -334,4 +332,4 @@ function localEventToGoogle(event) {
   return gEvent;
 }
 
-module.exports = { getAuthUrl, handleCallback, getStatus, disconnect, sync };
+export { getAuthUrl, handleCallback, getStatus, disconnect, sync };

server/services/recurrence.js

@@ -5,8 +5,6 @@
  * Abhängigkeiten: keine
  */
 
-'use strict';
-
 const DAY_MAP = { MO: 1, TU: 2, WE: 3, TH: 4, FR: 5, SA: 6, SU: 0 };
 
 /**
@@ -107,4 +105,4 @@ function nextOccurrence(baseDateStr, rrule) {
   return next.toISOString().slice(0, 10); // YYYY-MM-DD
 }
 
-module.exports = { parseRRule, nextOccurrence };
+export { parseRRule, nextOccurrence };