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

tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundApiOptionsValidatorTests.cs

@@ -83,4 +83,38 @@ public class InboundApiOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("ApiMethodCacheTtl", result.FailureMessage);
     }
+
+    // --- Standby forwarding (2026-08-28) ---
+
+    [Fact]
+    public void StandbyForwardPeerEndpoints_AbsoluteHttpUrls_AreValid()
+    {
+        var result = Validate(new InboundApiOptions
+        {
+            StandbyForwardPeerEndpoints = { "http://central-b:5000", "https://central-b.example/" },
+        });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Theory]
+    [InlineData("central-b:5000")]
+    [InlineData("/api")]
+    [InlineData("ftp://central-b")]
+    [InlineData("")]
+    public void StandbyForwardPeerEndpoints_NonHttpOrRelative_IsRejected(string peer)
+    {
+        var result = Validate(new InboundApiOptions { StandbyForwardPeerEndpoints = { peer } });
+
+        Assert.True(result.Failed);
+        Assert.Contains("StandbyForwardPeerEndpoints", result.FailureMessage);
+    }
+
+    [Fact]
+    public void ZeroStandbyForwardTimeout_IsRejected()
+    {
+        var result = Validate(new InboundApiOptions { StandbyForwardTimeout = TimeSpan.Zero });
+
+        Assert.True(result.Failed);
+        Assert.Contains("StandbyForwardTimeout", result.FailureMessage);
+    }
 }