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.Worker/SerilogRegistrar.cs

@@ -0,0 +1,21 @@
+using Microsoft.Extensions.Logging;
+using Serilog;
+
+namespace HrynCo.NotificationService.Worker;
+
+public static class SerilogRegistrar
+{
+    public static void AddSerilog(this HostApplicationBuilder builder)
+    {
+        var loggerConfiguration = new LoggerConfiguration()
+            .ReadFrom.Configuration(builder.Configuration)
+            .Enrich.FromLogContext();
+
+        Log.Logger = loggerConfiguration.CreateLogger();
+
+        builder.Logging.AddSerilog(Log.Logger);
+        builder.Services.AddSerilog();
+
+        builder.Services.AddSingleton(Log.Logger);
+    }
+}