fix(store-and-forward): resolve S&F delivery + replication wiring (3 Critical findings)

Resolves StoreAndForward-001, ExternalSystemGateway-001, NotificationService-001
— one systemic gap where buffered messages were persisted but never delivered,
and the active node never replicated its buffer to the standby.

Delivery handlers (ExternalSystemGateway-001 / NotificationService-001):
- AkkaHostedService registers delivery handlers for the ExternalSystem,
  CachedDbWrite and Notification categories after StoreAndForwardService starts;
  each resolves its scoped consumer in a fresh DI scope.
- ExternalSystemClient, DatabaseGateway and NotificationDeliveryService each
  gain a DeliverBufferedAsync method: re-resolve the target and re-attempt
  delivery, returning true/false/throwing per the transient-vs-permanent contract.
- EnqueueAsync gains an attemptImmediateDelivery flag; CachedCallAsync and
  NotificationDeliveryService.SendAsync pass false (they already attempted
  delivery themselves) so registering a handler does not dispatch twice.

Replication (StoreAndForward-001):
- ReplicationService is injected into StoreAndForwardService; a new BufferAsync
  helper replicates every enqueue, and successful-retry removes and parks are
  replicated too. Fire-and-forget, no-op when replication is disabled.

Tests: StoreAndForwardReplicationTests (Add/Remove/Park observed),
attemptImmediateDelivery behaviour, and DeliverBufferedAsync paths for each
consumer. Full solution builds; StoreAndForward/ExternalSystemGateway/
NotificationService suites green.

code-reviews/NotificationService/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-16 |
 | Reviewer | claude-agent |
 | Commit reviewed | `9c60592` |
-| Open findings | 12 |
+| Open findings | 11 |
 
 ## Summary
 
@@ -53,7 +53,7 @@ fallback in `DeliverAsync`, and concurrency on the token cache.
 |--|--|
 | Severity | Critical |
 | Category | Error handling & resilience |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.NotificationService/NotificationDeliveryService.cs:96`, `src/ScadaLink.NotificationService/ServiceCollectionExtensions.cs:8` |
 
 **Description**
@@ -66,7 +66,15 @@ Register a delivery handler for `StoreAndForwardCategory.Notification` during st
 
 **Resolution**
 
-_Unresolved._
+Resolved 2026-05-16. A delivery handler for `StoreAndForwardCategory.Notification` is now
+registered at site startup in `AkkaHostedService`. The handler resolves
+`NotificationDeliveryService` in a fresh DI scope and calls the new `DeliverBufferedAsync`,
+which re-resolves the list, recipients and SMTP config and re-attempts delivery —
+returning `true` on success, `false` (park) on permanent failure or missing
+configuration, and throwing on transient failure so the engine retries. `SendAsync` now
+buffers with `attemptImmediateDelivery: false` so registering the handler does not send
+the notification twice. Regression tests cover the happy path and the list-removed park
+path. Fixed by the commit whose message references `NotificationService-001`.
 
 ### NotificationService-002 — `TimeoutException`/`OperationCanceledException` misclassified as transient