fix(host): /health/active = oldest member + DB reachable — partition-safe Traefik routing

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

@@ -0,0 +1,31 @@
+using Akka.Actor;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Health;
+
+/// <summary>
+/// /health/active check: Healthy only on the oldest Up member — the node that
+/// hosts the cluster singletons. Replaces the shared package's leader-based
+/// ActiveNodeHealthCheck (review 01 [High]): leadership is address-ordered and
+/// diverges from singleton placement after a restart, and during a partition
+/// BOTH nodes compute themselves leader, so Traefik served both. Oldest-member
+/// semantics keep exactly one active node through a partition (the non-oldest
+/// side still sees the old oldest member until SBR resolves membership).
+/// </summary>
+public sealed class OldestNodeActiveHealthCheck : IHealthCheck
+{
+    private readonly ActorSystem _system;
+
+    /// <summary>Initializes a new <see cref="OldestNodeActiveHealthCheck"/> bound to the running actor system.</summary>
+    /// <param name="system">The live cluster actor system.</param>
+    public OldestNodeActiveHealthCheck(ActorSystem system) => _system = system;
+
+    /// <inheritdoc />
+    public Task<HealthCheckResult> CheckHealthAsync(HealthCheckContext context, CancellationToken ct = default)
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system);
+        return Task.FromResult(ClusterActivityEvaluator.SelfIsOldest(cluster)
+            ? HealthCheckResult.Healthy("oldest Up member (singleton host)")
+            : HealthCheckResult.Unhealthy("not the oldest Up member (standby)"));
+    }
+}