feat(host): register Audit Log #23 singletons with dedicated dispatcher (#23)

Wires Bundle E of the M2 site-sync pipeline:

- AddAuditLog extended to register the site writer chain (SqliteAuditWriter
  singleton + ISiteAuditQueue forward + RingBufferFallback + FallbackAuditWriter
  composing them) and the telemetry collaborators (SiteAuditTelemetryOptions,
  SqliteAuditWriterOptions, IAuditWriteFailureCounter NoOp default,
  ISiteStreamAuditClient NoOp default).
- AkkaHostedService central role: AuditLogIngestActor as ClusterSingletonManager
  (singleton name 'audit-log-ingest') + ClusterSingletonProxy, mirroring the
  Notification Outbox pattern. Proxy is offered to SiteStreamGrpcServer if it
  resolves (Site path only today; M6 reconciliation will host gRPC on central).
- AkkaHostedService site role: SiteAuditTelemetryActor (per-site, NOT a
  singleton because each site is its own cluster), bound to a dedicated
  audit-telemetry-dispatcher (ForkJoinDispatcher, 2 dedicated threads).
- Program.cs + SiteServiceRegistration.Configure call AddAuditLog on both roles.
- AuditLogIngestActor gains a second constructor that takes IServiceProvider so
  the cluster singleton can create a fresh scope per message — IAuditLogRepository
  is a scoped EF Core service and cannot be pre-resolved from the root. The
  IAuditLogRepository constructor remains for Bundle D's MSSQL-fixture tests.

NoOp ISiteStreamAuditClient is deliberate: no site→central gRPC channel exists
in M2 (sites talk to central via Akka ClusterClient; gRPC SiteStreamService is
hosted on sites for central→site streaming). M6 reconciliation introduces the
real gRPC site→central client + central-hosted gRPC server. Bundle H's
integration test substitutes a stub client directly via the actor's Props.

Tests:
- tests/ScadaLink.AuditLog.Tests/AddAuditLogTests.cs — 11 tests (was 3): writer
  singleton, IAuditWriter as FallbackAuditWriter, ISiteAuditQueue same-instance
  as SqliteAuditWriter, options bind round-trip, NoOp default assertions.
- tests/ScadaLink.Host.Tests/AkkaHostedServiceAuditWiringTests.cs (new) — 13
  tests: BuildHocon emits audit-telemetry-dispatcher block with the expected
  type/throughput/thread-count; Central composition root resolves the writer
  chain + options; Site composition root resolves the writer chain + options +
  NoOp client.

Verified: dotnet build clean, 23 test suites green (Host 194 + AuditLog 54).

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -128,6 +128,13 @@ public class AkkaHostedService : IHostedService
         var rolesStr = string.Join(",", roles.Select(QuoteHocon));
 
         return $@"
+audit-telemetry-dispatcher {{
+    type = ForkJoinDispatcher
+    throughput = 100
+    dedicated-thread-pool {{
+        thread-count = 2
+    }}
+}}
 akka {{
     extensions = [
         ""Akka.Cluster.Tools.PublishSubscribe.DistributedPubSubExtensionProvider, Akka.Cluster.Tools""
@@ -294,6 +301,47 @@ akka {{
         commService?.SetNotificationOutbox(outboxProxy);
         _logger.LogInformation("NotificationOutbox singleton created and registered with CentralCommunicationActor");
 
+        // Audit Log (#23) — central singleton mirrors the Notification Outbox
+        // pattern. The IngestAuditEvents gRPC handler lives on SiteStreamGrpcServer
+        // (Communication.Grpc); a central node hosting that server (M6 reconciliation
+        // path) hands the proxy in via SetAuditIngestActor below. When the gRPC
+        // server is not registered (current central topology), the host still
+        // brings the singleton up so a Bundle H in-process test (or a future
+        // direct caller) can Ask the proxy without further wiring.
+        // IAuditLogRepository is a SCOPED EF Core service, so the singleton
+        // actor takes the root IServiceProvider and creates a fresh scope per
+        // message (mirroring NotificationOutboxActor). Pre-resolving the
+        // repository here would attempt to take a scoped service from the
+        // root and fail under DI scope validation.
+        var auditIngestLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
+            .CreateLogger<ScadaLink.AuditLog.Central.AuditLogIngestActor>();
+
+        var auditIngestSingletonProps = ClusterSingletonManager.Props(
+            singletonProps: Props.Create(() => new ScadaLink.AuditLog.Central.AuditLogIngestActor(
+                _serviceProvider,
+                auditIngestLogger)),
+            terminationMessage: PoisonPill.Instance,
+            settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
+                .WithSingletonName("audit-log-ingest"));
+        _actorSystem!.ActorOf(auditIngestSingletonProps, "audit-log-ingest-singleton");
+
+        var auditIngestProxyProps = ClusterSingletonProxy.Props(
+            singletonManagerPath: "/user/audit-log-ingest-singleton",
+            settings: ClusterSingletonProxySettings.Create(_actorSystem)
+                .WithSingletonName("audit-log-ingest"));
+        var auditIngestProxy = _actorSystem.ActorOf(auditIngestProxyProps, "audit-log-ingest-proxy");
+
+        // Hand the proxy to the SiteStreamGrpcServer (if registered on this node)
+        // so the IngestAuditEvents RPC routes incoming site batches to the singleton.
+        // The gRPC server is currently only registered on Site nodes; on a central
+        // node this resolves to null and the wiring is a no-op until M6 (which
+        // brings central-hosted gRPC + a real site→central client).
+        var grpcServer = _serviceProvider.GetService<ScadaLink.Communication.Grpc.SiteStreamGrpcServer>();
+        grpcServer?.SetAuditIngestActor(auditIngestProxy);
+        _logger.LogInformation(
+            "AuditLogIngestActor singleton created (gRPC server bound: {GrpcBound})",
+            grpcServer is not null);
+
         _logger.LogInformation("Central actors registered. CentralCommunicationActor created.");
     }
 
@@ -504,6 +552,41 @@ akka {{
                 contacts.Count, _nodeOptions.SiteId);
         }
 
+        // Audit Log (#23) — site-side telemetry actor that drains the SQLite
+        // Pending queue and pushes to central via IngestAuditEvents. Not a
+        // cluster singleton: each site is its own cluster, and the actor reads
+        // node-local SQLite (no replication). The Props are bound to the
+        // dedicated audit-telemetry-dispatcher (defined in BuildHocon) so a
+        // batch SQLite read + gRPC push never contend with the default
+        // dispatcher used by hot-path actors.
+        //
+        // Per Bundle E's brief: the SiteAuditTelemetryActor takes its
+        // collaborators through its constructor, so we resolve them from DI
+        // and pass them in via Props.Create rather than relying on a future
+        // FactoryProvider. This also lets the M6 follow-up swap the
+        // NoOpSiteStreamAuditClient registration for the real gRPC client
+        // without touching this site wiring.
+        var siteAuditOptions = _serviceProvider
+            .GetRequiredService<IOptions<ScadaLink.AuditLog.Site.Telemetry.SiteAuditTelemetryOptions>>();
+        var siteAuditQueue = _serviceProvider
+            .GetRequiredService<ScadaLink.AuditLog.Site.Telemetry.ISiteAuditQueue>();
+        var siteAuditClient = _serviceProvider
+            .GetRequiredService<ScadaLink.AuditLog.Site.Telemetry.ISiteStreamAuditClient>();
+        var siteAuditLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
+            .CreateLogger<ScadaLink.AuditLog.Site.Telemetry.SiteAuditTelemetryActor>();
+
+        var siteAuditTelemetryProps = Props.Create(() =>
+                new ScadaLink.AuditLog.Site.Telemetry.SiteAuditTelemetryActor(
+                    siteAuditQueue,
+                    siteAuditClient,
+                    siteAuditOptions,
+                    siteAuditLogger))
+            .WithDispatcher("audit-telemetry-dispatcher");
+        _actorSystem.ActorOf(siteAuditTelemetryProps, "site-audit-telemetry");
+        _logger.LogInformation(
+            "SiteAuditTelemetryActor created (dispatcher=audit-telemetry-dispatcher, client={ClientType})",
+            siteAuditClient.GetType().Name);
+
         // Gate gRPC subscriptions until the actor system and SiteStreamManager are
         // initialized (REQ-HOST-7).
         //

src/ScadaLink.Host/Program.cs

@@ -1,5 +1,6 @@
 using HealthChecks.UI.Client;
 using Microsoft.AspNetCore.Diagnostics.HealthChecks;
+using ScadaLink.AuditLog;
 using ScadaLink.CentralUI;
 using ScadaLink.ClusterInfrastructure;
 using ScadaLink.Communication;
@@ -77,6 +78,10 @@ try
         // AddNotificationService() SMTP machinery above. AddNotificationOutbox binds
         // NotificationOutboxOptions via BindConfiguration, so no explicit Configure is needed.
         builder.Services.AddNotificationOutbox();
+        // Audit Log (#23) — central node owns the AuditLogIngestActor singleton +
+        // IAuditLogRepository. The site writer chain is still registered (lazy
+        // singletons) but is never resolved on a central node.
+        builder.Services.AddAuditLog(builder.Configuration);
         builder.Services.AddTemplateEngine();
         builder.Services.AddDeploymentManager();
         builder.Services.AddSecurity();

src/ScadaLink.Host/ScadaLink.Host.csproj

@@ -38,6 +38,7 @@
     <ProjectReference Include="../ScadaLink.ExternalSystemGateway/ScadaLink.ExternalSystemGateway.csproj" />
     <ProjectReference Include="../ScadaLink.NotificationService/ScadaLink.NotificationService.csproj" />
     <ProjectReference Include="../ScadaLink.NotificationOutbox/ScadaLink.NotificationOutbox.csproj" />
+    <ProjectReference Include="../ScadaLink.AuditLog/ScadaLink.AuditLog.csproj" />
     <ProjectReference Include="../ScadaLink.CentralUI/ScadaLink.CentralUI.csproj" />
     <ProjectReference Include="../ScadaLink.Security/ScadaLink.Security.csproj" />
     <ProjectReference Include="../ScadaLink.HealthMonitoring/ScadaLink.HealthMonitoring.csproj" />

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -1,3 +1,4 @@
+using ScadaLink.AuditLog;
 using ScadaLink.ClusterInfrastructure;
 using ScadaLink.Communication;
 using ScadaLink.DataConnectionLayer;
@@ -44,6 +45,12 @@ public static class SiteServiceRegistration
         services.AddStoreAndForward();
         services.AddSiteEventLogging();
 
+        // Audit Log (#23) — site-side hot-path writer + telemetry collaborators.
+        // The SiteAuditTelemetryActor itself is registered by AkkaHostedService
+        // in the site-role block; this call wires every DI dependency it (and
+        // ScriptRuntimeContext, when Bundle F lands) reaches for.
+        services.AddAuditLog(config);
+
         // WP-13: Akka.NET bootstrap via hosted service
         services.AddSingleton<AkkaHostedService>();
         services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());