Guard writer units against activation and fix backup cancellation

usr-local-sbin/app-restore.sh

@@ -164,6 +164,7 @@ quiesce
 MUTATION_STARTED=true
 save_recovery
 for i in "${!SELECTED[@]}"; do
+  assert_services_quiescent
   label=${SELECTED[i]}
   source_path="$STAGING_DIR/extracted/$label/${SELECTED_ROOTS[i]}"
   if [[ ${IS_DB[i]} == true ]]; then
@@ -175,6 +176,7 @@ for i in "${!SELECTED[@]}"; do
     rsync -aHAX --numeric-ids "${EXCLUDES[@]}" -- "$source_path/" "${TARGETS[i]}/" 8>&- 9>&-
   fi
 done
+assert_services_quiescent
 # A backup captured maintenance=true. Enforce the recorded pre-restore state even
 # if the existing installation already was in maintenance mode.
 if [[ " ${SELECTED[*]} " == *' nextcloud '* || " ${SELECTED[*]} " == *' db-nextcloud '* ]]; then