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/Core/RequestHandler.cs

@@ -17,8 +17,10 @@ public abstract class RequestHandler<TRequest, TResponse> : IRequestHandler<TReq
     protected ILogger Logger { get; }
     protected IUnitOfWork UnitOfWork { get; }
 
-    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken) =>
-        DoOnHandle(request, cancellationToken);
+    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken)
+    {
+        return DoOnHandle(request, cancellationToken);
+    }
 
     protected abstract Task<TResponse> DoOnHandle(TRequest request, CancellationToken cancellationToken);
-}
+}