fix(setup): CompleteAsNode idempotent — konvertiert Standalone zu Cluster-Node

Wenn setup bereits abgeschlossen war (z.B. zuvor als Standalone eingerichtet),
wird der Node jetzt ohne Fehler auf is_cluster_node=true umgestellt statt
mit "setup already completed" zu fehlschlagen.

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

internal/services/setup/setup.go

@@ -168,6 +168,10 @@ func (s *Store) Complete(req Request) (*State, error) {
 // CompleteAsNode configures this node as a secondary cluster member.
 // No admin credentials are created; login becomes available after PG
 // replication brings users from the primary.
+//
+// Idempotent: if setup was already completed (e.g. the node was
+// previously configured as standalone), it is converted to cluster-node
+// mode. Calling again after a successful join is a no-op.
 func (s *Store) CompleteAsNode(req NodeRequest) (*State, error) {
 	if !looksLikeFQDN(req.FQDN) {
 		return nil, fmt.Errorf("fqdn %q does not look like a fully-qualified hostname", req.FQDN)
@@ -179,16 +183,22 @@ func (s *Store) CompleteAsNode(req NodeRequest) (*State, error) {
 	if err != nil {
 		return nil, err
 	}
-	if prev.Completed {
-		return nil, errors.New("setup already completed")
-	}
 	now := time.Now().UTC()
+	completedAt := &now
+	if prev.CompletedAt != nil {
+		completedAt = prev.CompletedAt
+	}
 	st := &State{
 		FQDN:          strings.TrimSpace(req.FQDN),
 		ACMEEmail:     strings.ToLower(strings.TrimSpace(req.ACMEEmail)),
 		IsClusterNode: true,
 		Completed:     true,
-		CompletedAt:   &now,
+		CompletedAt:   completedAt,
+		// Carry over non-auth fields from previous state.
+		LicenseKey:          prev.LicenseKey,
+		IPv6Enabled:         prev.IPv6Enabled,
+		BackupRetentionKeep: prev.BackupRetentionKeep,
+		AuditRetentionDays:  prev.AuditRetentionDays,
 	}
 	if err := s.Save(st); err != nil {
 		return nil, err