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/Logging/IContextualSerilogLogger.cs

@@ -0,0 +1,11 @@
+using System.Diagnostics.CodeAnalysis;
+using Serilog;
+
+namespace HrynCo.NotificationService.Services.Logging;
+
+[SuppressMessage("Major Code Smell", "S2326:Unused type parameters should be removed",
+    Justification = "Generic parameter used in implementation via ForContext<T>.")]
+public interface IContextualSerilogLogger<TContext>
+{
+    ILogger Logger { get; }
+}