feat: replace manual Stopwatch with IProfiler in TransactionBehavior

- Add HrynCo.Common to Services project
- TransactionBehavior now uses IProfiler.MeasureExecutionAsync:
  MeasureExecutionAsync -> ExecuteInTransactionAsync -> next() -> SaveChangesAsync
- Profiler logs Start/End with duration + memory delta via Serilog PerformanceLog context
- Register IProfiler as singleton in ServiceCollectionExtensions (uses Log.Logger)

Ref: IT-628

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

HrynCo.NotificationService.Services/ServiceCollectionExtensions.cs

@@ -1,7 +1,9 @@
+using HrynCo.Common;
 using HrynCo.NotificationService.Services.Behaviors;
 using HrynCo.NotificationService.Services.Logging;
 using MediatR;
 using Microsoft.Extensions.DependencyInjection;
+using Serilog;
 
 namespace HrynCo.NotificationService.Services;
 
@@ -16,6 +18,7 @@ public static class ServiceCollectionExtensions
         });
 
         services.AddTransient(typeof(IContextualSerilogLogger<>), typeof(ContextualSerilogLogger<>));
+        services.AddSingleton<IProfiler>(new Profiler(Log.Logger));
 
         return services;
     }