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).

code-reviews/NotificationService/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-28 |
 | Reviewer | claude-agent |
 | Commit reviewed | `1eb6e97` |
-| Open findings | 7 |
+| Open findings | 6 |
 
 ## Summary
 
@@ -683,9 +683,11 @@ Recommended path is option 1: the parallel implementation in `EmailNotificationD
 |--|--|
 | Severity | Medium |
 | Category | Correctness & logic bugs |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.Host/Actors/AkkaHostedService.cs:654-660`, NS-001 resolution note (this file) |
 
+**Resolution (2026-05-28):** Added a `_notificationDeliveryHandlerRegistered` sentinel field on `AkkaHostedService` and gated the canonical `NotificationForwarder` registration with an `InvalidOperationException` guard — a future code path that re-introduces the dead NS-001 site-SMTP handler now fails fast at startup with an explicit NS-020 diagnostic, rather than silently overwriting `RegisterDeliveryHandler`'s last-write-wins map and inverting the central-only design. The sentinel's XML doc cross-references NS-001/NS-019/NS-020 so a maintainer searching for the `Notification` S&F handler finds the one canonical registration and its history.
+
 **Description**
 
 NS-001 was resolved by registering an `S&F → DeliverBufferedAsync` handler for `StoreAndForwardCategory.Notification` at site startup in `AkkaHostedService`. The current source registers a **different** handler for the same category at `AkkaHostedService.cs:654-660` — `NotificationForwarder.DeliverAsync`, which forwards to central instead of sending SMTP. `StoreAndForwardService.RegisterDeliveryHandler` (verified by reading `StoreAndForward/StoreAndForwardService.cs` around line 109) takes a single handler per category — last-write-wins or first-write-wins, either way the two registrations cannot both be active.