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

docker/central-node-a/appsettings.Central.json

@@ -76,7 +76,9 @@
       "OfflineTimeout": "00:01:00"
     },
     "InboundApi": {
-      "DefaultMethodTimeout": "00:00:30"
+      "DefaultMethodTimeout": "00:00:30",
+      "_standbyForwarding": "2026-08-28 — a standby central node relays POST /api/{methodName} to the active peer instead of answering 503 STANDBY_NODE. List the OTHER node only.",
+      "StandbyForwardPeerEndpoints": [ "http://scadabridge-central-b:5000" ]
     },
     "Notification": {
       "SmtpServer": "scadabridge-smtp",

docker/central-node-b/appsettings.Central.json

@@ -76,7 +76,9 @@
       "OfflineTimeout": "00:01:00"
     },
     "InboundApi": {
-      "DefaultMethodTimeout": "00:00:30"
+      "DefaultMethodTimeout": "00:00:30",
+      "_standbyForwarding": "2026-08-28 — a standby central node relays POST /api/{methodName} to the active peer instead of answering 503 STANDBY_NODE. List the OTHER node only.",
+      "StandbyForwardPeerEndpoints": [ "http://scadabridge-central-a:5000" ]
     },
     "Notification": {
       "SmtpServer": "scadabridge-smtp",