Make backup marker symlinks relative

scripts/backup.sh

@@ -178,10 +178,12 @@ finalize_backup() {
 
     log_info "Backup ownership updated"
 
+    RELATIVE_BACKUP_DIR="./$(basename $BACKUP_DIR)"
+
     docker compose run --rm init sh -c '
         ln -sfn "$1" /backups/.latest_tmp &&
         mv -Tf /backups/.latest_tmp /backups/latest
-    ' -- "$BACKUP_DIR" || {
+    ' -- "$RELATIVE_BACKUP_DIR" || {
         log_error "Failed to update /backups/latest symlink"
         return 1
     }