feat: wire ExternalSystem, Database, and Notify APIs into script runtime

IServiceProvider now flows through the actor chain (DeploymentManagerActor
→ InstanceActor → ScriptActor → ScriptExecutionActor) so scripts can
resolve IExternalSystemClient, IDatabaseGateway, and
INotificationDeliveryService from DI. ScriptGlobals exposes ExternalSystem,
Database, Notify, and Scripts as top-level properties so scripts can use
them without the Instance. prefix.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -241,7 +241,8 @@ akka {{
                 siteRuntimeOptionsValue,
                 dmLogger,
                 dclManager,
-                siteHealthCollector)),
+                siteHealthCollector,
+                _serviceProvider)),
             terminationMessage: PoisonPill.Instance,
             settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
                 .WithRole(siteRole)