fix(inbound-api): resolve InboundAPI-002,004,006,008 — disconnect vs timeout, body size limit, active-node gate; surface InboundAPI-007

src/ScadaLink.InboundAPI/IActiveNodeGate.cs

@@ -0,0 +1,24 @@
+namespace ScadaLink.InboundAPI;
+
+/// <summary>
+/// InboundAPI-008: abstraction the inbound API endpoint uses to determine whether
+/// this node is the active (cluster-leader) central node.
+///
+/// The design states the inbound API is "Central cluster only (active node)" and
+/// "fails over with it". A standby central node must not execute method scripts or
+/// <c>Route.To()</c> calls — that can race the active node or run against stale
+/// singleton state. <see cref="InboundApiEndpointFilter"/> consults this gate and
+/// returns HTTP 503 from a standby so Traefik/clients only reach the live node.
+///
+/// The implementation lives in the Host (it needs Akka cluster state); when no
+/// implementation is registered, the endpoint defaults to "allow" so non-clustered
+/// hosts and tests are unaffected.
+/// </summary>
+public interface IActiveNodeGate
+{
+    /// <summary>
+    /// <c>true</c> when this node is the active central node and may serve the
+    /// inbound API; <c>false</c> on a standby node.
+    /// </summary>
+    bool IsActiveNode { get; }
+}