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/Middleware/AuditWriteMiddleware.cs

@@ -219,6 +219,12 @@ public sealed class AuditWriteMiddleware
         {
             sw.Stop();
 
+            // Standby forwarding (2026-08-28): a request this node RELAYED to the active
+            // peer is audited there, by the node that actually authenticated and executed
+            // it. Emitting a second row here would double-count every relayed call, so the
+            // standby skips its own row when the filter flagged the relay.
+            var relayed = ctx.Items.ContainsKey(InboundApiEndpointFilter.RelayedToActiveItemKey);
+
             // Restore the original stream and resolve the captured audit copy.
             // The forwarding wrapper has already written every byte to the
             // original sink; this just pulls back the bounded UTF-8 string.
@@ -232,7 +238,8 @@ public sealed class AuditWriteMiddleware
             var responseBody = skipBody ? null : capturedResponseBody;
             var responseTruncated = skipBody ? false : capturedResponseTruncated;
 
-            EmitInboundAudit(
+            if (!relayed)
+                EmitInboundAudit(
                 ctx,
                 opts,
                 sw.ElapsedMilliseconds,