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/termine.js

@@ -1,6 +1,7 @@
 import { promises as fs } from 'fs'
 import path from 'path'
 import { randomUUID } from 'crypto'
+import { writeDataFileWithRotation } from './data-file-rotation.js'
 
 // Use internal server/data directory for Termine CSV to avoid writing to public/
 const getDataPath = (filename) => {
@@ -89,7 +90,7 @@ export async function writeTermine(termine) {
       csv += `"${escapedDatum}","${escapedUhrzeit}","${escapedTitel}","${escapedBeschreibung}","${escapedKategorie}"\n`
     }
     
-    await fs.writeFile(TERMINE_FILE, csv, 'utf-8')
+    await writeDataFileWithRotation(TERMINE_FILE, csv, { encoding: 'utf-8' })
     return true
   } catch (error) {
     console.error('Fehler beim Schreiben der Termine:', error)