fix(site-event-logging): resolve SiteEventLogging-005,007,008,010 — background async writer, drop concrete downcast, surface write failures, test coverage

src/ScadaLink.SiteEventLogging/ServiceCollectionExtensions.cs

@@ -9,7 +9,13 @@ public static class ServiceCollectionExtensions
     /// </summary>
     public static IServiceCollection AddSiteEventLogging(this IServiceCollection services)
     {
-        services.AddSingleton<ISiteEventLogger, SiteEventLogger>();
+        // The recorder is registered as a concrete singleton and the interface is
+        // forwarded to the same instance. The purge and query services depend on the
+        // concrete SiteEventLogger directly (they need its lock-guarded WithConnection)
+        // rather than downcasting an ISiteEventLogger, which would throw
+        // InvalidCastException for any other ISiteEventLogger implementation.
+        services.AddSingleton<SiteEventLogger>();
+        services.AddSingleton<ISiteEventLogger>(sp => sp.GetRequiredService<SiteEventLogger>());
         services.AddSingleton<IEventLogQueryService, EventLogQueryService>();
         services.AddHostedService<EventLogPurgeService>();
         return services;