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

src/ScadaLink.SiteEventLogging/EventLogPurgeService.cs

@@ -19,12 +19,14 @@ public class EventLogPurgeService : BackgroundService
     private readonly ILogger<EventLogPurgeService> _logger;
 
     public EventLogPurgeService(
-        ISiteEventLogger eventLogger,
+        SiteEventLogger eventLogger,
         IOptions<SiteEventLogOptions> options,
         ILogger<EventLogPurgeService> logger)
     {
-        // We need the concrete type to funnel access through its shared lock.
-        _eventLogger = (SiteEventLogger)eventLogger;
+        // Depend on the concrete recorder directly: purge must funnel database access
+        // through its lock-guarded WithConnection. Taking ISiteEventLogger and
+        // downcasting would throw InvalidCastException for any other implementation.
+        _eventLogger = eventLogger;
         _options = options.Value;
         _logger = logger;
     }