feat: add SerilogRegistrar and ContextualSerilogLogger

- IContextualSerilogLogger<T> / ContextualSerilogLogger<T> in Services/Logging
  (handlers get a ForContext<T>-scoped logger via DI, consistent with ItemTracker)
- SerilogRegistrar extension on WebApplicationBuilder (Api)
- SerilogRegistrar extension on HostApplicationBuilder (Worker)
- Both registrars: set Log.Logger, wire Logging + Host/Services, register ILogger singleton
- ServiceCollectionExtensions: register IContextualSerilogLogger<> as transient
- Program.cs in both apps simplified to single builder.AddSerilog() call

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Services/ServiceCollectionExtensions.cs

@@ -1,4 +1,5 @@
 using HrynCo.NotificationService.Services.Behaviors;
+using HrynCo.NotificationService.Services.Logging;
 using MediatR;
 using Microsoft.Extensions.DependencyInjection;
 
@@ -14,6 +15,8 @@ public static class ServiceCollectionExtensions
             cfg.AddBehavior(typeof(IPipelineBehavior<,>), typeof(TransactionBehavior<,>));
         });
 
+        services.AddTransient(typeof(IContextualSerilogLogger<>), typeof(ContextualSerilogLogger<>));
+
         return services;
     }
 }