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.

docker/central-node-a/appsettings.Central.json

@@ -64,6 +64,12 @@
       "DispatchInterval": "00:00:05",
       "DispatchBatchSize": 1000
     },
+    "KpiHistory": {
+      "SampleInterval": "00:01:00",
+      "RetentionDays": 90,
+      "PurgeInterval": "1.00:00:00",
+      "DefaultMaxSeriesPoints": 200
+    },
     "Transport": {
       "SourceEnvironment": "docker-cluster"
     },