fix: reduce USB backup retention from 30 to 15 files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/Backend/src/cron/backupCheck.ts

@@ -11,7 +11,7 @@ const LOCAL_BACKUP_DIR = path.resolve(process.cwd(), "backups");
 // Name of the USB backup subfolder the user creates on their drive
 const USB_BACKUP_FOLDER_NAME = "USB Backup";
 
-const MAX_BACKUPS = 30;
+const MAX_BACKUPS = 15;
 
 function ensureLocalBackupDir() {
   if (!fs.existsSync(LOCAL_BACKUP_DIR)) {