feat(alerts): add alert for failed systemd services (#2173)

Adds a user-configurable "Failed Services" alert that notifies when any
tracked systemd service enters the failed state, and again when all services
recover.

---------

Signed-off-by: Martin Stenröse <martin@stenrose.se>
Co-authored-by: henrygd <hank@henrygd.me>

internal/alerts/alerts_test_helpers.go

@@ -88,6 +88,10 @@ func ResolveStatusAlerts(app core.App) error {
 	return resolveStatusAlerts(app)
 }
 
+func ResolveSystemdAlerts(app core.App) error {
+	return resolveSystemdAlerts(app)
+}
+
 func (am *AlertManager) RestorePendingStatusAlerts() error {
 	return am.restorePendingStatusAlerts()
 }