Fix nil pointer panic when notifier Send returns nil result

Guard against nil result before accessing result.Error in processNotification,
and add NtfyNotifier.Validate override so DefaultTopic is considered before
rejecting notifications with zero recipients.

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

internal/service/service.go

@@ -243,9 +243,11 @@ func (s *NotificationService) processNotification(ctx context.Context, msg *doma
 
 	// Send the notification
 	result, err := notifier.Send(ctx, notification)
-	if err != nil || !result.Success {
+	if err != nil || result == nil || !result.Success {
 		notification.RetryCount++
-		notification.LastError = result.Error
+		if result != nil {
+			notification.LastError = result.Error
+		}
 		if err != nil {
 			notification.LastError = err.Error()
 		}