Add unit tests for data file rotation utility functions

- Implement tests for writing data files with rotation, ensuring backups are created only on changes.
- Verify that old backups are rotated correctly and the maximum number of backups is maintained.
- Test restoration of backups while preserving the current state as a backup.
- Utilize Vitest for testing framework and manage temporary file storage during tests.

server/utils/newsletter.js

@@ -4,6 +4,7 @@ import { readMembers } from './members.js'
 import { readUsers } from './auth.js'
 import { encryptObject, decryptObject } from './encryption.js'
 import crypto from 'crypto'
+import { writeDataFileWithRotation } from './data-file-rotation.js'
 
 // nosemgrep: javascript.lang.security.audit.path-traversal.path-join-resolve-traversal.path-join-resolve-traversal
 // filename is always a hardcoded constant (e.g., 'newsletter-subscribers.json'), never user input
@@ -136,7 +137,7 @@ export async function writeSubscribers(subscribers) {
   try {
     const encryptionKey = getEncryptionKey()
     const encryptedData = encryptObject(subscribers, encryptionKey)
-    await fs.writeFile(NEWSLETTER_SUBSCRIBERS_FILE, encryptedData, 'utf-8')
+    await writeDataFileWithRotation(NEWSLETTER_SUBSCRIBERS_FILE, encryptedData, { encoding: 'utf-8' })
     return true
   } catch (error) {
     console.error('Fehler beim Schreiben der Newsletter-Abonnenten:', error)