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/InboundApiOptionsValidator.cs

@@ -31,5 +31,18 @@ public sealed class InboundApiOptionsValidator : OptionsValidatorBase<InboundApi
         builder.RequireThat(options.ApiMethodCacheTtl > TimeSpan.Zero,
             $"ScadaBridge:InboundApi:ApiMethodCacheTtl must be a positive duration " +
             $"(was {options.ApiMethodCacheTtl}); it bounds how long a resolved ApiMethod row is cached.");
+
+        builder.RequireThat(options.StandbyForwardTimeout > TimeSpan.Zero,
+            $"ScadaBridge:InboundApi:StandbyForwardTimeout must be a positive duration " +
+            $"(was {options.StandbyForwardTimeout}); it bounds each standby→active relay attempt.");
+
+        foreach (var peer in options.StandbyForwardPeerEndpoints)
+        {
+            var ok = Uri.TryCreate(peer, UriKind.Absolute, out var uri)
+                && (uri.Scheme == Uri.UriSchemeHttp || uri.Scheme == Uri.UriSchemeHttps);
+            builder.RequireThat(ok,
+                $"ScadaBridge:InboundApi:StandbyForwardPeerEndpoints entry '{peer}' must be an absolute " +
+                $"http/https URL (e.g. http://central-b:5000); it is the peer a standby relays inbound API calls to.");
+        }
     }
 }