feat(inbound-api): standby central node relays POST /api/* to the active peer instead of 503

A caller addressing a standby central node directly (or hitting Traefik inside its
/health/active probe window after a failover) used to get 503 STANDBY_NODE. The
standby now relays the request verbatim to the active peer over HTTP
(IStandbyRequestForwarder / HttpStandbyRequestForwarder) and replays the response;
method scripts and Route.To() still execute only on the active node.

- ScadaBridge:InboundApi:StandbyForwardPeerEndpoints (static list of the OTHER
  central node's HTTP base URL; empty = old 503 behaviour) + StandbyForwardTimeout
  (60s), both startup-validated.
- Peers tried in order: peer 503 stamped X-ScadaBridge-Standby, connection failure
  or timeout → next peer; all fail → 503 ACTIVE_NODE_UNREACHABLE. A 503 without
  the marker is the active node's own answer and is replayed.
- One-hop loop guard via X-ScadaBridge-Forwarded; 413 body cap now runs BEFORE
  the active-node check so a standby never buffers/relays an oversized body.
- AuditWriteMiddleware skips its own ApiInbound row for a relayed request so a
  call is audited once, on the node that executed it.
- Rig + env2 central nodes configured; spec (Component-InboundAPI.md → Standby
  forwarding), Component-Host.md options row, CLAUDE.md updated.
- 20 new tests (filter decision + relay mechanics + validator).

Claude-Session: https://claude.ai/code/session_019ZxXT5sxZs5XrtdwYzJDPV

src/ZB.MOM.WW.ScadaBridge.InboundAPI/ServiceCollectionExtensions.cs

@@ -35,6 +35,15 @@ public static class ServiceCollectionExtensions
         // body size cap and active-node gating for POST /api/{methodName}.
         services.AddSingleton<InboundApiEndpointFilter>();
 
+        // Standby-node forwarding (2026-08-28): a standby relays POST /api/{methodName}
+        // to the active peer over HTTP instead of answering 503 STANDBY_NODE. The
+        // named client has no HttpClient.Timeout of its own — the forwarder's per-attempt
+        // CTS (InboundApiOptions.StandbyForwardTimeout) is the single deadline, so a
+        // long-running method on the active node is not cut off by a second, hidden cap.
+        services.AddHttpClient(HttpStandbyRequestForwarder.HttpClientName)
+            .ConfigureHttpClient(c => c.Timeout = Timeout.InfiniteTimeSpan);
+        services.AddSingleton<IStandbyRequestForwarder, HttpStandbyRequestForwarder>();
+
         // N1: the ApiMethod consumer of the script-artifact invalidation bus.
         // The bus parameter is optional — hosts without a registered
         // IScriptArtifactChangeBus (site roles, plain test hosts) start it as a no-op