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_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()