Add support for notification dry run feature

- Introduced NOTIFICATION_DRY_RUN configuration option in .env.example and k8s/secret.example.yaml.
- Updated README.md to include usage instructions for the new dry run feature.
- Implemented logic in app.go to preview notifications without sending them when the dry run option is enabled.
- Enhanced config.go to load the new configuration option and validate notification types accordingly.
- Added a new function in notify.go to generate manual-needed notification messages for preview.

internal/app/app.go

@@ -122,6 +122,11 @@ func Run(cfg config.Config) (model.RunSummary, error) {
 		return summary, err
 	}
 
+	if cfg.NotificationDryRun {
+		printNotificationPreview(cfg.NotificationType, summary.Results)
+		return summary, nil
+	}
+
 	notifier := newNotifier(cfg)
 	if notifier != nil {
 		if err := notifier.SendManualNeeded(summary.Results); err != nil {
@@ -300,3 +305,17 @@ func newNotifier(cfg config.Config) notify.Sender {
 		return nil
 	}
 }
+
+func printNotificationPreview(notificationType string, results []model.ActionResult) {
+	subject, body, ok := notify.ManualNeededMessage(results)
+	if !ok {
+		fmt.Fprintln(os.Stderr, "notification dry-run: no manual-needed torrents, nothing would be sent")
+		return
+	}
+
+	if notificationType == "" {
+		notificationType = "unconfigured"
+	}
+	fmt.Fprintf(os.Stderr, "notification dry-run: type=%s\n", notificationType)
+	fmt.Fprintf(os.Stderr, "Subject: %s\n\n%s", subject, body)
+}