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.

tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundApiOptionsValidatorTests.cs

@@ -65,4 +65,22 @@ public class InboundApiOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("MaxRequestBodyBytes", result.FailureMessage);
     }
+
+    [Fact]
+    public void ZeroApiMethodCacheTtl_IsRejected()
+    {
+        var result = Validate(new InboundApiOptions { ApiMethodCacheTtl = TimeSpan.Zero });
+
+        Assert.True(result.Failed);
+        Assert.Contains("ApiMethodCacheTtl", result.FailureMessage);
+    }
+
+    [Fact]
+    public void NegativeApiMethodCacheTtl_IsRejected()
+    {
+        var result = Validate(new InboundApiOptions { ApiMethodCacheTtl = TimeSpan.FromSeconds(-1) });
+
+        Assert.True(result.Failed);
+        Assert.Contains("ApiMethodCacheTtl", result.FailureMessage);
+    }
 }