fix(archreview): security authz+hub cluster (SEC-07/05/08/11) + validation hardening

SEC-07: add QueryActiveAlarmsRequest -> events:read scope arm; fix two tests that
  constructed StreamAlarmsRequest instead of QueryActiveAlarmsRequest.
SEC-05: shorten hub-token lifetime 30m -> 5m; document that the ?access_token= query
  carriage must never be request-logged.
SEC-08: gateway-side CachingApiKeyVerifier (short TTL, keyed on a hash of the presented
  secret) skips the per-call store read+last_used write; CoalescingMarkApiKeyStore bounds
  last_used writes to <=1/key/min; identity constraints are cached. Invalidation is wired
  at the gateway admin sites (revoke/rotate/delete); short TTL backstops out-of-process CLI.
SEC-11: fixed-window rate limit on POST /auth/login + a per-peer (key-id) failure limiter
  checked before VerifyAsync; new MxGateway:Security options bound + validated.

Also fixes regressions from the SEC-01/04/06 commit (c185f62) that a narrow test filter
missed (all now covered by a full-suite checkpoint):
- Rooting check is cross-platform: accepts Windows C:\/UNC forms on Unix so the shipped
  appsettings path validates on the macOS dev box, still rejecting bare filenames.
- AddGatewayConfiguration TryAdds a non-production IHostEnvironment fallback so the validator
  resolves in minimal test/tooling containers; the real host + apikey CLI register the actual
  environment first (TryAdd no-op there).
- Test assembly defaults ASPNETCORE_ENVIRONMENT=Development (ModuleInitializer) so full-host
  tests exercise wiring instead of tripping the SEC-04/06 production guards.
- GatewayOptionsTests asserts the SEC-01 CommonApplicationData-derived default (platform-correct).

archreview: SEC-07/05/08/11 (P1). Verified: NonWindows build clean; full gateway suite
747 passed / 42 failed, where all 42 are the pre-existing macOS named-pipe-harness failures
(Unix-socket path limit) and 0 are validation/regression failures.

docs/Metrics.md

@@ -203,6 +203,18 @@ metrics.RecordEventStreamSend(publicEvent.Family.ToString(), stopwatch.Elapsed);
 
 `Dashboard/DashboardSnapshotService.cs` calls `_metrics.GetSnapshot()` once per `GetSnapshot` invocation and projects it into the dashboard transport types together with the session registry view. The dashboard receives a single, internally consistent snapshot per tick rather than reading individual counters at separate times. See [Gateway Dashboard Design](./GatewayDashboardDesign.md) and [Dashboard Interface Design](./DashboardInterfaceDesign.md) for the projection rules and wire format.
 
+## Auth Store Write Churn
+
+The per-RPC authentication path is a hidden write source: the shared verifier
+refreshes `last_used_utc` on every authenticated call. To keep that off the
+auth-store throughput ceiling, the gateway caches successful verifications for a
+short TTL and coalesces the `last_used_utc` write to at most one per key per
+window (`MxGateway:Security:ApiKeyVerificationCacheSeconds` /
+`ApiKeyLastUsedCoalesceSeconds`, defaults 15 s / 60 s). This bounds WAL churn on
+the bulk-read workload without dropping any metric — auth activity is still
+observable via command-throughput counters and audit rows. See
+[Authentication](./Authentication.md#hot-path-caching-and-last-used-coalescing).
+
 ## Related Documentation
 
 - [Gateway Dashboard Design](./GatewayDashboardDesign.md)