nextcloud: make mail sync idempotent

scripts/nextcloud-mail-sync.sh

@@ -15,7 +15,14 @@ account_exists() {
   local email="${2}"
 
   # Nextcloud Mail does not provide a list command; export is safe (does not print passwords).
-  /usr/sbin/runuser -u www-data -- php occ mail:account:export "${user_id}" 2>/dev/null | grep -Fq -- "- E-Mail: ${email}"
+  local export
+  if ! export=$(/usr/sbin/runuser -u www-data -- php occ mail:account:export "${user_id}" 2>/dev/null); then
+    echo "WARN: unable to export mail accounts for ${user_id}; skipping sync for safety" >&2
+    return 0
+  fi
+
+  # Output formatting varies by Nextcloud/Mail versions and locale; match by email address.
+  grep -Fq -- "${email}" <<<"${export}"
 }
 
 token=$(