Phase 3A: Site runtime foundation — Akka cluster, SQLite persistence, Deployment Manager singleton, Instance Actor

- WP-1: Site cluster config (keep-oldest SBR, down-if-alone, 2s/10s failure detection)
- WP-2: Site-role host bootstrap (no Kestrel, SQLite paths)
- WP-3: SiteStorageService with deployed_configurations + static_attribute_overrides tables
- WP-4: DeploymentManagerActor as cluster singleton with staggered Instance Actor creation,
  OneForOneStrategy/Resume supervision, deploy/disable/enable/delete lifecycle
- WP-5: InstanceActor with attribute state, GetAttribute/SetAttribute, SQLite override persistence
- WP-6: CoordinatedShutdown verified for graceful singleton handover
- WP-7: Dual-node recovery (both seed nodes, min-nr-of-members=1)
- WP-8: 31 tests (storage CRUD, actor lifecycle, supervision, negative checks)
389 total tests pass, zero warnings.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -1,8 +1,13 @@
 using Akka.Actor;
+using Akka.Cluster;
+using Akka.Cluster.Tools.Singleton;
 using Akka.Configuration;
 using Microsoft.Extensions.Options;
 using ScadaLink.ClusterInfrastructure;
 using ScadaLink.Host.Actors;
+using ScadaLink.SiteRuntime;
+using ScadaLink.SiteRuntime.Actors;
+using ScadaLink.SiteRuntime.Persistence;
 
 namespace ScadaLink.Host.Actors;
 
@@ -41,6 +46,10 @@ public class AkkaHostedService : IHostedService
         var seedNodesStr = string.Join(",",
             _clusterOptions.SeedNodes.Select(s => $"\"{s}\""));
 
+        // For site nodes, include a site-specific role (e.g., "site-SiteA") alongside the base role
+        var roles = BuildRoles();
+        var rolesStr = string.Join(",", roles.Select(r => $"\"{r}\""));
+
         var hocon = $@"
 akka {{
     actor {{
@@ -54,7 +63,7 @@ akka {{
     }}
     cluster {{
         seed-nodes = [{seedNodesStr}]
-        roles = [""{_nodeOptions.Role}""]
+        roles = [{rolesStr}]
         min-nr-of-members = {_clusterOptions.MinNrOfMembers}
         split-brain-resolver {{
             active-strategy = {_clusterOptions.SplitBrainResolverStrategy}
@@ -78,8 +87,9 @@ akka {{
         _actorSystem = ActorSystem.Create("scadalink", config);
 
         _logger.LogInformation(
-            "Akka.NET actor system 'scadalink' started. Role={Role}, Hostname={Hostname}, Port={Port}",
+            "Akka.NET actor system 'scadalink' started. Role={Role}, Roles={Roles}, Hostname={Hostname}, Port={Port}",
             _nodeOptions.Role,
+            string.Join(", ", roles),
             _nodeOptions.NodeHostname,
             _nodeOptions.RemotingPort);
 
@@ -90,6 +100,12 @@ akka {{
             Props.Create(() => new DeadLetterMonitorActor(dlmLogger)),
             "dead-letter-monitor");
 
+        // For site nodes, register the Deployment Manager as a cluster singleton
+        if (_nodeOptions.Role.Equals("Site", StringComparison.OrdinalIgnoreCase))
+        {
+            RegisterSiteActors();
+        }
+
         return Task.CompletedTask;
     }
 
@@ -103,4 +119,62 @@ akka {{
             _logger.LogInformation("Akka.NET actor system shutdown complete.");
         }
     }
+
+    /// <summary>
+    /// Builds the list of cluster roles for this node. Site nodes get both "Site"
+    /// and a site-specific role (e.g., "site-SiteA") to scope singleton placement.
+    /// </summary>
+    private List<string> BuildRoles()
+    {
+        var roles = new List<string> { _nodeOptions.Role };
+
+        if (_nodeOptions.Role.Equals("Site", StringComparison.OrdinalIgnoreCase)
+            && !string.IsNullOrEmpty(_nodeOptions.SiteId))
+        {
+            roles.Add($"site-{_nodeOptions.SiteId}");
+        }
+
+        return roles;
+    }
+
+    /// <summary>
+    /// Registers site-specific actors including the Deployment Manager cluster singleton.
+    /// The singleton is scoped to the site-specific cluster role so it runs on exactly
+    /// one node within this site's cluster.
+    /// </summary>
+    private void RegisterSiteActors()
+    {
+        var siteRole = $"site-{_nodeOptions.SiteId}";
+        var storage = _serviceProvider.GetRequiredService<SiteStorageService>();
+        var siteRuntimeOptionsValue = _serviceProvider.GetService<IOptions<SiteRuntimeOptions>>()?.Value
+            ?? new SiteRuntimeOptions();
+        var dmLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
+            .CreateLogger<DeploymentManagerActor>();
+
+        // Create the Deployment Manager as a cluster singleton
+        var singletonProps = ClusterSingletonManager.Props(
+            singletonProps: Props.Create(() => new DeploymentManagerActor(
+                storage,
+                siteRuntimeOptionsValue,
+                dmLogger)),
+            terminationMessage: PoisonPill.Instance,
+            settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
+                .WithRole(siteRole)
+                .WithSingletonName("deployment-manager"));
+
+        _actorSystem!.ActorOf(singletonProps, "deployment-manager-singleton");
+
+        // Create a proxy for other actors to communicate with the singleton
+        var proxyProps = ClusterSingletonProxy.Props(
+            singletonManagerPath: "/user/deployment-manager-singleton",
+            settings: ClusterSingletonProxySettings.Create(_actorSystem)
+                .WithRole(siteRole)
+                .WithSingletonName("deployment-manager"));
+
+        _actorSystem.ActorOf(proxyProps, "deployment-manager-proxy");
+
+        _logger.LogInformation(
+            "Site actors registered. DeploymentManager singleton scoped to role={SiteRole}",
+            siteRole);
+    }
 }