fix(siteeventlog): gate EventLogPurge to active node via IClusterNodeProvider.SelfIsPrimary (#29, M2.15)

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

@@ -96,6 +96,19 @@ public static class SiteServiceRegistration
             return new AkkaClusterNodeProvider(akkaService, siteRole);
         });
 
+        // SiteEventLogging-019 / #29 (M2.15): 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
+        // never call SiteServiceRegistration leave it unregistered, so the purge defaults
+        // to always-run (the pre-fix behaviour, preserved).
+        services.AddSingleton<SiteEventLogActiveNodeCheck>(sp =>
+        {
+            var nodeProvider = sp.GetRequiredService<IClusterNodeProvider>();
+            return () => nodeProvider.SelfIsPrimary;
+        });
+
         // Options binding
         BindSharedOptions(services, config);
         services.Configure<SiteRuntimeOptions>(config.GetSection("ScadaBridge:SiteRuntime"));