feat(kpi): K5 — Host central wiring + KpiHistoryRecorder cluster singleton + appsettings (not readiness-gated)

Wire the M6 KPI History recorder into the central composition path:
- Program.cs: call services.AddKpiHistory(configuration) on the central-only
  branch alongside AddNotificationOutbox/AddAuditLog/AddSiteCallAudit.
- AkkaHostedService.cs: register KpiHistoryRecorderActor as a central,
  non-role-scoped ClusterSingletonManager + ClusterSingletonProxy + a
  PhaseClusterLeave CoordinatedShutdown graceful-stop drain (singleton name
  'kpi-history-recorder'), copied/adapted from the audit-log-purge block.
- appsettings.Central.json (Host + docker + docker-env2 central nodes): add a
  ScadaBridge:KpiHistory section (SampleInterval 00:01:00, RetentionDays 90,
  PurgeInterval 1.00:00:00, DefaultMaxSeriesPoints 200).

KPI history is observability/best-effort and MUST NOT gate readiness: the
recorder is deliberately NOT added to RequiredSingletonsHealthCheck or any
other readiness gate.

src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs

@@ -699,6 +699,63 @@ akka {{
         _actorSystem.ActorOf(auditReconProxyProps, "site-audit-reconciliation-proxy");
         _logger.LogInformation("SiteAuditReconciliationActor singleton created");
 
+        // KPI History (#26, M6) — central singleton that periodically samples the
+        // Notification Outbox / Site Call Audit point-in-time KPIs into the
+        // KpiHistorySamples table and runs the daily retention purge. Mirrors the
+        // audit-log-purge singleton pattern above: a ClusterSingletonManager pins
+        // the recorder to the active central node, a ClusterSingletonProxy gives a
+        // stable address, and a PhaseClusterLeave graceful-stop task drains the
+        // in-flight tick before handover. The recorder's sample + purge timers
+        // self-schedule in PreStart. Options come from AddKpiHistory (central
+        // composition root only). The actor takes the root IServiceProvider and
+        // opens its own per-tick DI scope (the KPI repository is a scoped EF Core
+        // service), so the 3 ctor args (IServiceProvider, KpiHistoryOptions,
+        // ILogger) are resolved here from DI exactly like the other singletons.
+        // NOT readiness-gated by design: KPI history is observability/best-effort
+        // (it must never gate /health/ready), so kpi-history-recorder is
+        // deliberately absent from RequiredSingletonsHealthCheck.
+        var kpiHistoryOptions = _serviceProvider
+            .GetRequiredService<IOptions<ZB.MOM.WW.ScadaBridge.KpiHistory.KpiHistoryOptions>>().Value;
+        var kpiHistoryLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
+            .CreateLogger<ZB.MOM.WW.ScadaBridge.KpiHistory.KpiHistoryRecorderActor>();
+
+        var kpiHistorySingletonProps = ClusterSingletonManager.Props(
+            singletonProps: Props.Create(() => new ZB.MOM.WW.ScadaBridge.KpiHistory.KpiHistoryRecorderActor(
+                _serviceProvider,
+                kpiHistoryOptions,
+                kpiHistoryLogger)),
+            terminationMessage: PoisonPill.Instance,
+            settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
+                .WithSingletonName("kpi-history-recorder"));
+        var kpiHistorySingletonManager =
+            _actorSystem!.ActorOf(kpiHistorySingletonProps, "kpi-history-recorder-singleton");
+
+        var kpiHistoryShutdown = Akka.Actor.CoordinatedShutdown.Get(_actorSystem);
+        kpiHistoryShutdown.AddTask(
+            Akka.Actor.CoordinatedShutdown.PhaseClusterLeave,
+            "drain-kpi-history-recorder-singleton",
+            async () =>
+            {
+                try
+                {
+                    await kpiHistorySingletonManager.GracefulStop(TimeSpan.FromSeconds(10));
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogWarning(ex,
+                        "KpiHistoryRecorder singleton did not drain within the graceful-stop "
+                        + "timeout; falling through to PoisonPill handover");
+                }
+                return Akka.Done.Instance;
+            });
+
+        var kpiHistoryProxyProps = ClusterSingletonProxy.Props(
+            singletonManagerPath: "/user/kpi-history-recorder-singleton",
+            settings: ClusterSingletonProxySettings.Create(_actorSystem)
+                .WithSingletonName("kpi-history-recorder"));
+        _actorSystem.ActorOf(kpiHistoryProxyProps, "kpi-history-recorder-proxy");
+        _logger.LogInformation("KpiHistoryRecorderActor singleton created (not readiness-gated)");
+
         _logger.LogInformation("Central actors registered. CentralCommunicationActor created.");
     }
 

src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -16,6 +16,7 @@ using ZB.MOM.WW.ScadaBridge.Host.Actors;
 using ZB.MOM.WW.ScadaBridge.Host.Health;
 using ZB.MOM.WW.ScadaBridge.InboundAPI;
 using ZB.MOM.WW.ScadaBridge.InboundAPI.Middleware;
+using ZB.MOM.WW.ScadaBridge.KpiHistory;
 using ZB.MOM.WW.ScadaBridge.ManagementService;
 using ZB.MOM.WW.ScadaBridge.NotificationOutbox;
 using ZB.MOM.WW.ScadaBridge.NotificationService;
@@ -110,6 +111,11 @@ try
         // but the call is here for symmetry with the other audit composition
         // roots so future per-actor DI lands without touching Program.cs.
         builder.Services.AddSiteCallAudit();
+        // KPI History (#26, M6) — central-only. Binds KpiHistoryOptions from
+        // ScadaBridge:KpiHistory and registers the validated options consumed by
+        // the KpiHistoryRecorderActor cluster singleton (started in
+        // AkkaHostedService). Observability/best-effort: NOT readiness-gated.
+        builder.Services.AddKpiHistory(builder.Configuration);
         builder.Services.AddTemplateEngine();
         builder.Services.AddDeploymentManager();
         // Host is the composition root and owns config-coupled wiring: register the

src/ZB.MOM.WW.ScadaBridge.Host/appsettings.Central.json

@@ -64,6 +64,12 @@
       "PurgeInterval": "1.00:00:00",
       "DeliveredKpiWindow": "00:01:00"
     },
+    "KpiHistory": {
+      "SampleInterval": "00:01:00",
+      "RetentionDays": 90,
+      "PurgeInterval": "1.00:00:00",
+      "DefaultMaxSeriesPoints": 200
+    },
     "Transport": {
       "BundleSessionTtlMinutes": 30,
       "MaxBundleSizeMb": 100,