fix(admin): SMTP port can be cleared and saves without 'Ungültiges JSON'

The UI sent the port as a number while the server expects text, so every
save failed; the field also jumped back to 587 when emptied. The port is
now edited as text (digits only), and the server accepts number or text.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/domain/models.go

@@ -3,6 +3,7 @@ package domain
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"encoding/json"
 	"fmt"
 	"regexp"
 	"strings"
@@ -289,6 +290,36 @@ type SystemSettings struct {
 	SMTPConfiguredViaEnv bool   `json:"smtpConfiguredViaEnv,omitempty"`
 }
 
+// UnmarshalJSON accepts smtpPort as text or number: older clients sent a number, which made the whole
+// settings request fail with "Ungültiges JSON".
+func (s *SystemSettings) UnmarshalJSON(data []byte) error {
+	type plain SystemSettings
+	aux := struct {
+		*plain
+		SMTPPort json.RawMessage `json:"smtpPort"`
+	}{plain: (*plain)(s)}
+	if err := json.Unmarshal(data, &aux); err != nil {
+		return err
+	}
+	port := strings.TrimSpace(string(aux.SMTPPort))
+	switch {
+	case port == "" || port == "null":
+	case strings.HasPrefix(port, `"`):
+		var str string
+		if err := json.Unmarshal(aux.SMTPPort, &str); err != nil {
+			return err
+		}
+		s.SMTPPort = strings.TrimSpace(str)
+	default:
+		var n json.Number
+		if err := json.Unmarshal(aux.SMTPPort, &n); err != nil {
+			return err
+		}
+		s.SMTPPort = n.String()
+	}
+	return nil
+}
+
 type DocumentBlock struct {
 	ID         string  `json:"id"`
 	DocumentID string  `json:"documentId,omitempty"`

internal/domain/tags_test.go

@@ -1,6 +1,7 @@
 package domain
 
 import (
+	"encoding/json"
 	"reflect"
 	"testing"
 )
@@ -32,3 +33,19 @@ func TestStaleAnchorTags(t *testing.T) {
 		t.Fatalf("only tags that exist solely as anchors are stale, got %v", got)
 	}
 }
+
+func TestSystemSettingsAcceptsPortAsNumberOrText(t *testing.T) {
+	for _, body := range []string{`{"smtpPort": 465, "smtpHost": "mail"}`, `{"smtpPort": "465", "smtpHost": "mail"}`} {
+		var s SystemSettings
+		if err := json.Unmarshal([]byte(body), &s); err != nil {
+			t.Fatalf("%s: %v", body, err)
+		}
+		if s.SMTPPort != "465" || s.SMTPHost != "mail" {
+			t.Errorf("%s: got port %q host %q", body, s.SMTPPort, s.SMTPHost)
+		}
+	}
+	var empty SystemSettings
+	if err := json.Unmarshal([]byte(`{"smtpPort": ""}`), &empty); err != nil || empty.SMTPPort != "" {
+		t.Errorf("empty port: %q (%v)", empty.SMTPPort, err)
+	}
+}