fix(host): unify active-node on oldest-member semantics — purge/self-report/labels now track the singleton host

src/ZB.MOM.WW.ScadaBridge.Host/Health/ActiveNodeGate.cs

@@ -9,9 +9,12 @@ namespace ZB.MOM.WW.ScadaBridge.Host.Health;
 /// <see cref="IActiveNodeGate"/> backed by the running Akka.NET cluster.
 ///
 /// The inbound API is "Central cluster only (active node)" — a standby central
-/// node must not execute method scripts or <c>Route.To()</c> calls. This gate
-/// mirrors the leadership check in <see cref="ActiveNodeHealthCheck"/> (the
-/// node is the cluster leader, <see cref="MemberStatus.Up"/>), so
+/// node must not execute method scripts or <c>Route.To()</c> calls. Active =
+/// the oldest Up member (the singleton host), evaluated by
+/// <see cref="ClusterActivityEvaluator.SelfIsOldest(Cluster, string?)"/> — NOT
+/// the cluster leader, whose address-ordered definition diverges from singleton
+/// placement after a restart (review 01 [High]). This is the same evaluator
+/// backing <see cref="OldestNodeActiveHealthCheck"/>, so
 /// <see cref="InboundApiEndpointFilter"/> can return HTTP 503 on a standby.
 ///
 /// Registered only in the Central-role branch of <c>Program.cs</c>. The gate
@@ -42,12 +45,7 @@ public sealed class ActiveNodeGate : IActiveNodeGate
                 return false;
 
             var cluster = Cluster.Get(system);
-            var self = cluster.SelfMember;
-            if (self.Status != MemberStatus.Up)
-                return false;
-
-            var leader = cluster.State.Leader;
-            return leader != null && leader == self.Address;
+            return ClusterActivityEvaluator.SelfIsOldest(cluster);
         }
     }
 }

src/ZB.MOM.WW.ScadaBridge.Host/Health/AkkaClusterNodeProvider.cs

@@ -33,9 +33,7 @@ public class AkkaClusterNodeProvider : IClusterNodeProvider
             var system = _akkaService.ActorSystem;
             if (system == null) return false;
             var cluster = Cluster.Get(system);
-            if (cluster.SelfMember.Status != MemberStatus.Up) return false;
-            var leader = cluster.State.Leader;
-            return leader != null && leader.Equals(cluster.SelfAddress);
+            return ClusterActivityEvaluator.SelfIsOldest(cluster, _siteRole);
         }
     }
 
@@ -46,8 +44,7 @@ public class AkkaClusterNodeProvider : IClusterNodeProvider
         if (system == null) return [];
 
         var cluster = Cluster.Get(system);
-        var selfAddress = cluster.SelfAddress;
-        var leader = cluster.State.Leader;
+        var oldest = ClusterActivityEvaluator.OldestUpMember(cluster, _siteRole);
 
         var nodes = new List<NodeStatus>();
         foreach (var member in cluster.State.Members)
@@ -57,8 +54,8 @@ public class AkkaClusterNodeProvider : IClusterNodeProvider
 
             var hostname = member.Address.Host ?? member.Address.ToString();
             var isOnline = member.Status == MemberStatus.Up;
-            var isLeader = member.Address.Equals(leader);
-            var role = isLeader ? "Primary" : "Standby";
+            var isPrimary = oldest != null && member.UniqueAddress.Equals(oldest.UniqueAddress);
+            var role = isPrimary ? "Primary" : "Standby";
 
             nodes.Add(new NodeStatus(hostname, isOnline, role));
         }

src/ZB.MOM.WW.ScadaBridge.Host/SiteServiceRegistration.cs

@@ -109,8 +109,8 @@ public static class SiteServiceRegistration
         // The EventLogPurgeService runs on every
         // site host node but consults this optional gate each tick and early-exits on
         // the standby. Register it to delegate to IClusterNodeProvider.SelfIsPrimary
-        // (the canonical "this node is Up AND cluster leader" check) so purge runs ONLY
-        // on the active node — no duplicated cluster logic. Non-clustered test hosts that
+        // (the canonical "this node is the oldest Up member (singleton host)" check) so purge
+        // runs ONLY on the active node — no duplicated cluster logic. Non-clustered test hosts that
         // never call SiteServiceRegistration leave it unregistered, so the purge defaults
         // to always-run (the pre-fix behaviour, preserved).
         services.AddSingleton<SiteEventLogActiveNodeCheck>(sp =>