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/EndpointExtensions.cs

@@ -91,6 +91,7 @@ public static class EndpointExtensions
         var logger = httpContext.RequestServices.GetRequiredService<ILogger<InboundApiEndpoint>>();
         var verifier = httpContext.RequestServices.GetRequiredService<IApiKeyVerifier>();
         var repository = httpContext.RequestServices.GetRequiredService<IInboundApiRepository>();
+        var methodCache = httpContext.RequestServices.GetRequiredService<ApiMethodCache>();
         var executor = httpContext.RequestServices.GetRequiredService<InboundScriptExecutor>();
         var routeHelper = httpContext.RequestServices.GetRequiredService<RouteHelper>();
         var options = httpContext.RequestServices.GetRequiredService<IOptions<InboundApiOptions>>().Value;
@@ -150,9 +151,17 @@ public static class EndpointExtensions
         // "Echo" does not grant "echo". This is the intended invariant: method
         // names are case-sensitive identifiers, and a key's granted scopes must be
         // provisioned with the exact casing of the methods they authorize.
+        // WP2.6b: the per-request repository round-trip is fronted by a short-TTL
+        // ApiMethodCache (invalidated by name on an ApiMethod ScriptArtifactsChanged
+        // notification, and self-healing via TTL expiry otherwise — see ApiMethodCache
+        // and ScriptArtifactChangeSubscriber). Only reached after the in-scope check
+        // passes, preserving the existing enumeration-safety ordering below.
         var inScope = identity.Scopes.Contains(methodName);
         var method = inScope
-            ? await repository.GetMethodByNameAsync(methodName, httpContext.RequestAborted)
+            ? await methodCache.GetOrFetchAsync(
+                methodName,
+                ct => repository.GetMethodByNameAsync(methodName, ct),
+                httpContext.RequestAborted)
             : null;
 
         if (method == null || !inScope)