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.InboundAPI/InboundApiOptions.cs

@@ -34,4 +34,18 @@ public class InboundApiOptions
     /// </para>
     /// </summary>
     public string ApiKeyPepper { get; set; } = string.Empty;
+
+    /// <summary>
+    /// WP2.6b (arch-review misc — Inbound API per-request SQL): how long a resolved
+    /// <c>ApiMethod</c> row (found OR not-found) is served from <see cref="ApiMethodCache"/>
+    /// before the next request re-fetches it from the repository. The
+    /// <c>ScriptArtifactChangeSubscriber</c>/<c>IScriptArtifactChangeBus</c> pipeline
+    /// invalidates a specific method's entry immediately on a bundle-import change (the fast
+    /// path); this TTL is the self-healing fallback for changes the bus does not cover (e.g.
+    /// an operator edit via the Management API/UI, which does not publish a bus notification),
+    /// bounding staleness without requiring every mutation path to remember to invalidate.
+    /// Short by design — the whole point is to absorb repeated-request-storm SQL, not to
+    /// serve minutes-old method definitions.
+    /// </summary>
+    public TimeSpan ApiMethodCacheTtl { get; set; } = TimeSpan.FromSeconds(5);
 }