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/config/config_test.go

@@ -55,6 +55,7 @@ func TestNew_DefaultValues(t *testing.T) {
 	assert.Empty(t, cfg.SentryDSN)
 	assert.Empty(t, cfg.MetricsUsername)
 	assert.Empty(t, cfg.MetricsPassword)
+	assert.False(t, cfg.SendTestNotification)
 }
 
 func TestNew_EnvironmentOverrides(t *testing.T) {
@@ -73,6 +74,7 @@ func TestNew_EnvironmentOverrides(t *testing.T) {
 	t.Setenv("DNSWATCHER_MAINTENANCE_MODE", "true")
 	t.Setenv("DNSWATCHER_METRICS_USERNAME", "admin")
 	t.Setenv("DNSWATCHER_METRICS_PASSWORD", "secret")
+	t.Setenv("DNSWATCHER_SEND_TEST_NOTIFICATION", "true")
 
 	cfg, err := config.New(nil, newTestParams(t))
 	require.NoError(t, err)
@@ -90,6 +92,7 @@ func TestNew_EnvironmentOverrides(t *testing.T) {
 	assert.True(t, cfg.MaintenanceMode)
 	assert.Equal(t, "admin", cfg.MetricsUsername)
 	assert.Equal(t, "secret", cfg.MetricsPassword)
+	assert.True(t, cfg.SendTestNotification)
 }
 
 func TestNew_NoTargetsError(t *testing.T) {