feat: add DNSWATCHER_SEND_TEST_NOTIFICATION env var

When set to a truthy value, sends a startup status notification
to all configured notification channels after the first full scan
completes. The notification is clearly an all-ok/success message
showing the number of monitored domains, hostnames, ports, and
certificates.

Closes #84

internal/watcher/watcher.go

@@ -129,6 +129,7 @@ func (w *Watcher) Run(ctx context.Context) {
 	)
 
 	w.RunOnce(ctx)
+	w.maybeSendTestNotification(ctx)
 
 	dnsTicker := time.NewTicker(w.config.DNSInterval)
 	tlsTicker := time.NewTicker(w.config.TLSInterval)
@@ -854,6 +855,38 @@ func (w *Watcher) saveState() {
 	}
 }
 
+// maybeSendTestNotification sends a startup status notification
+// after the first full scan completes, if SEND_TEST_NOTIFICATION
+// is enabled. The message is clearly informational ("all ok")
+// and not an error or anomaly alert.
+func (w *Watcher) maybeSendTestNotification(ctx context.Context) {
+	if !w.config.SendTestNotification {
+		return
+	}
+
+	snap := w.state.GetSnapshot()
+
+	msg := fmt.Sprintf(
+		"dnswatcher has started and completed its initial scan.\n"+
+			"Monitoring %d domain(s) and %d hostname(s).\n"+
+			"Tracking %d port endpoint(s) and %d TLS certificate(s).\n"+
+			"All notification channels are working.",
+		len(snap.Domains),
+		len(snap.Hostnames),
+		len(snap.Ports),
+		len(snap.Certificates),
+	)
+
+	w.log.Info("sending startup test notification")
+
+	w.notify.SendNotification(
+		ctx,
+		"✅ dnswatcher startup complete",
+		msg,
+		"success",
+	)
+}
+
 // --- Utility functions ---
 
 func toSet(items []string) map[string]bool {

internal/watcher/watcher_test.go

@@ -756,6 +756,117 @@ func TestDNSRunsBeforePortAndTLSChecks(t *testing.T) {
 	}
 }
 
+func TestSendTestNotification_Enabled(t *testing.T) {
+	t.Parallel()
+
+	cfg := defaultTestConfig(t)
+	cfg.Domains = []string{"example.com"}
+	cfg.Hostnames = []string{"www.example.com"}
+	cfg.SendTestNotification = true
+
+	w, deps := newTestWatcher(t, cfg)
+	setupBaselineMocks(deps)
+
+	w.RunOnce(t.Context())
+
+	// RunOnce does not send the test notification — it is
+	// sent by Run after RunOnce completes. Call the exported
+	// RunOnce then check that no test notification was sent
+	// (only Run triggers it). We test the full path via Run.
+	notifications := deps.notifier.getNotifications()
+	if len(notifications) != 0 {
+		t.Errorf(
+			"RunOnce should not send test notification, got %d",
+			len(notifications),
+		)
+	}
+}
+
+func TestSendTestNotification_ViaRun(t *testing.T) {
+	t.Parallel()
+
+	cfg := defaultTestConfig(t)
+	cfg.Domains = []string{"example.com"}
+	cfg.Hostnames = []string{"www.example.com"}
+	cfg.SendTestNotification = true
+	cfg.DNSInterval = 24 * time.Hour
+	cfg.TLSInterval = 24 * time.Hour
+
+	w, deps := newTestWatcher(t, cfg)
+	setupBaselineMocks(deps)
+
+	ctx, cancel := context.WithCancel(t.Context())
+
+	done := make(chan struct{})
+
+	go func() {
+		w.Run(ctx)
+		close(done)
+	}()
+
+	// Wait for the initial scan and test notification.
+	time.Sleep(500 * time.Millisecond)
+	cancel()
+
+	<-done
+
+	notifications := deps.notifier.getNotifications()
+
+	found := false
+
+	for _, n := range notifications {
+		if n.Priority == "success" &&
+			n.Title == "✅ dnswatcher startup complete" {
+			found = true
+		}
+	}
+
+	if !found {
+		t.Errorf(
+			"expected startup test notification, got: %v",
+			notifications,
+		)
+	}
+}
+
+func TestSendTestNotification_Disabled(t *testing.T) {
+	t.Parallel()
+
+	cfg := defaultTestConfig(t)
+	cfg.Domains = []string{"example.com"}
+	cfg.Hostnames = []string{"www.example.com"}
+	cfg.SendTestNotification = false
+	cfg.DNSInterval = 24 * time.Hour
+	cfg.TLSInterval = 24 * time.Hour
+
+	w, deps := newTestWatcher(t, cfg)
+	setupBaselineMocks(deps)
+
+	ctx, cancel := context.WithCancel(t.Context())
+
+	done := make(chan struct{})
+
+	go func() {
+		w.Run(ctx)
+		close(done)
+	}()
+
+	time.Sleep(500 * time.Millisecond)
+	cancel()
+
+	<-done
+
+	notifications := deps.notifier.getNotifications()
+
+	for _, n := range notifications {
+		if n.Title == "✅ dnswatcher startup complete" {
+			t.Error(
+				"test notification should not be sent when disabled",
+			)
+		}
+	}
+}
+
 func TestNSFailureAndRecovery(t *testing.T) {
 	t.Parallel()