fix(concurrency/lifetime): close Theme 5 — 10 concurrency / DI / scope findings

Concurrency hazards, DI lifetime hygiene, and one verify-only confirmation
across 8 modules. Highlights:

Concurrency:
- CentralUI-030: SandboxConsoleCapture writes routed through WriteSynchronized
  locking on the captured StringWriter — intra-script Task fan-out can no
  longer corrupt the per-call buffer.
- Commons-021: ExternalCallResult.Response now backed by Lazy<dynamic?>
  (ExecutionAndPublication) — no more benign double-parse race.
- CD-017: DeploymentManagerRepository.DeleteDeploymentRecordAsync now takes
  an expected RowVersion and seeds entry.OriginalValues so EF emits
  DELETE ... WHERE Id=@id AND RowVersion=@prior; stale RowVersion now
  throws DbUpdateConcurrencyException instead of silent overwrite.
- Transport-009: AuditCorrelationContext.BundleImportId backed by
  AsyncLocal<Guid?> so concurrent imports get per-logical-call isolation
  (was a scoped instance shared via AuditService across runs).

DI / lifetime:
- AuditLog-003: All 3 AuditLog actor handlers switched to CreateAsyncScope
  + await using — async EF disposal no longer swallowed.
- AuditLog-007: INodeIdentityProvider resolution standardised on
  GetRequiredService<>() (was mixed with GetService<>()).
- AuditLog-011: AddAuditLogHealthMetricsBridge guarded by sentinel
  descriptor check — calling twice no longer double-registers the hosted
  service.

Shutdown / supervision:
- SiteCallAudit-002: AkkaHostedService adds a CoordinatedShutdown
  cluster-leave task (drain-site-call-audit-singleton) that issues a
  bounded GracefulStop(10s) so failover waits for in-flight upserts.

Registration safety:
- NS-020: AkkaHostedService now guards NotificationForwarder S&F
  registration with _notificationDeliveryHandlerRegistered + throws
  InvalidOperationException on double-register to make the regression loud.

VERIFY-only closures:
- NotifOutbox-005: Confirmed already closed by CD-015 fix (ac96b83) —
  NotificationOutboxRepository.InsertIfNotExistsAsync uses the same
  raw-SQL IF NOT EXISTS + 2601/2627 swallow pattern; race eliminated.

5+ new regression tests (CentralUI sandbox WhenAll, ExternalCallResult
64-reader Barrier, AuditLog DI idempotency, RowVersion stale-throw,
SiteCallAudit-002 shutdown drain). Build clean; affected suites all green.
README regenerated: 65 open (was 75).

tests/ScadaLink.AuditLog.Tests/AddAuditLogTests.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
@@ -274,4 +275,36 @@ public class AddAuditLogTests
         Assert.Throws<InvalidOperationException>(
             () => provider.GetRequiredService<IAuditWriteFailureCounter>());
     }
+
+    [Fact]
+    public void AddAuditLogHealthMetricsBridge_IsIdempotent_DoesNotDoubleRegister_HostedService()
+    {
+        // AuditLog-011: AddHostedService has no TryAdd variant, so a second
+        // call without the sentinel guard would spin up a second
+        // SiteAuditBacklogReporter on the same SQLite file. The helper must
+        // be a no-op on the second call — exactly one hosted-service
+        // descriptor for SiteAuditBacklogReporter survives.
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["AuditLog:SiteWriter:DatabasePath"] = ":memory:",
+            })
+            .Build();
+
+        var services = new ServiceCollection();
+        services.AddSingleton<ILoggerFactory, NullLoggerFactory>();
+        services.AddSingleton(typeof(ILogger<>), typeof(NullLogger<>));
+        services.AddSingleton<INodeIdentityProvider>(new FakeNodeIdentityProvider());
+        services.AddAuditLog(config);
+        services.AddHealthMonitoring();
+
+        services.AddAuditLogHealthMetricsBridge();
+        services.AddAuditLogHealthMetricsBridge();
+
+        var reporterCount = services.Count(d =>
+            d.ServiceType == typeof(IHostedService) &&
+            d.ImplementationType == typeof(SiteAuditBacklogReporter));
+
+        Assert.Equal(1, reporterCount);
+    }
 }