perf(misc): cached hot-path lookups, bounded observer queue, alarm-priority stream path

WP2.6 (arch-review remediation, cross-cutting misc):
- SiteExternalSystemRepository: name/ID-indexed ExternalSystemDefinitionCache replaces
  the fetch-all + reverse-map scan on every by-ID/method lookup; loaded once per
  redeploy, invalidated by DeploymentManagerActor after HandleDeployArtifacts applies
  external-system changes. Static JsonSerializerOptions for method-list parsing.
- Inbound API: short-TTL ApiMethodCache fronts the per-request ApiMethod repository
  fetch; invalidated by name via the existing ScriptArtifactChangeSubscriber/
  IScriptArtifactChangeBus pipeline, self-healing via TTL for changes the bus
  doesn't cover (e.g. Management API edits).
- StoreAndForward: the cached-call audit-observer queue — the one unbounded channel
  left in the system — is now bounded (ObserverQueueCapacity, default 10,000) with
  DropOldest overflow and a dropped-notification counter.
- SiteStreamManager: alarm state changes now travel a dedicated publish
  source/broadcast hub, isolated from the (far higher-volume) attribute path, so an
  attribute storm can no longer evict a pending alarm transition; the alarm hand-off
  queue is bounded with a drop counter surfaced on the site health report
  (SiteStreamAlarmDropCount via the new SiteStreamAlarmDropReporter), and publishing
  is skipped entirely at zero subscribers on either path.
- CLI ManagementHttpClient: explicit 30s HttpClient.Timeout on the shared
  construction (was the 100s framework default), overridable via
  SCADABRIDGE_HTTP_TIMEOUT_SECONDS.

Deviation: the failback-probe heartbeat item is NOT included — its only viable
surface (CentralChannelProvider.cs / heartbeat consumers) lives entirely in the
Communication project, explicitly off-limits to this work package this phase.

Tests: SiteRuntime.Tests (550), InboundAPI.Tests (278), StoreAndForward.Tests (133),
CLI.Tests (390), HealthMonitoring.Tests (97) — all green after full solution build.

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardOptions.cs

@@ -38,4 +38,18 @@ public class StoreAndForwardOptions
     /// 1 = legacy serial. Within a lane delivery stays sequential (per-target FIFO).
     /// </summary>
     public int SweepTargetParallelism { get; set; } = 4;
+
+    /// <summary>
+    /// WP2.6c (arch-review misc — unbounded S&amp;F observer queue): capacity of the
+    /// cached-call audit-observer pump's queue. It was the one unbounded
+    /// <c>Channel&lt;T&gt;</c> left in the system — a slow/stuck
+    /// <c>ICachedCallLifecycleObserver</c> (a SQLite audit write) could not stretch
+    /// the retry sweep, but nothing stopped it growing without bound while the sweep kept
+    /// posting. Bounded with <see cref="System.Threading.Channels.BoundedChannelFullMode.DropOldest"/>
+    /// — the same overflow policy as its sibling bounded channels
+    /// (<c>SiteEventLogger</c>'s write queue, the debug/alarm stream hubs) — so a stuck
+    /// observer sheds the oldest unprocessed notifications instead of leaking memory; drops
+    /// are counted (see <c>StoreAndForwardService.ObserverQueueDroppedCount</c>).
+    /// </summary>
+    public int ObserverQueueCapacity { get; set; } = 10_000;
 }