fix(web-api): ensure graceful shutdown (#1784)

<!--- Provide a general summary of your changes in the Title above -->

## Description

<!--- Describe your changes in detail -->

Adjusting shutdown delay. Ref discussion in this thread:
https://digdir.slack.com/archives/C07R312076E/p1738364898052139

Plucked from this blogpost:
https://github.com/dotnet/dotnet-docker/blob/main/samples/kubernetes/graceful-shutdown/graceful-shutdown.md#adding-a-shutdown-delay

## Related Issue(s)

- #N/A

## Verification

- [ ] **Your** code builds clean without any errors or warnings
- [ ] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)

Author: arealmaas

src/Digdir.Domain.Dialogporten.WebApi/DelayedShutdownHostLifetime.cs

@@ -0,0 +1,47 @@
+using System.Runtime.InteropServices;
+
+namespace Digdir.Domain.Dialogporten.WebApi;
+
+public sealed class DelayedShutdownHostLifetime : IHostLifetime, IDisposable
+{
+    private readonly IHostApplicationLifetime _applicationLifetime;
+    private readonly TimeSpan _delay;
+    private IEnumerable<IDisposable>? _disposables;
+
+    public DelayedShutdownHostLifetime(IHostApplicationLifetime applicationLifetime, TimeSpan delay)
+    {
+        _applicationLifetime = applicationLifetime;
+        _delay = delay;
+    }
+
+    public Task StopAsync(CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+
+    public Task WaitForStartAsync(CancellationToken cancellationToken)
+    {
+        _disposables = new IDisposable[]
+        {
+            PosixSignalRegistration.Create(PosixSignal.SIGINT, HandleSignal),
+            PosixSignalRegistration.Create(PosixSignal.SIGQUIT, HandleSignal),
+            PosixSignalRegistration.Create(PosixSignal.SIGTERM, HandleSignal)
+        };
+        return Task.CompletedTask;
+    }
+
+    private void HandleSignal(PosixSignalContext ctx)
+    {
+        ctx.Cancel = true;
+        Task.Delay(_delay).ContinueWith(t => _applicationLifetime.StopApplication());
+    }
+
+    public void Dispose()
+    {
+        foreach (var disposable in _disposables ?? Enumerable.Empty<IDisposable>())
+        {
+            disposable.Dispose();
+        }
+        GC.SuppressFinalize(this);
+    }
+}

src/Digdir.Domain.Dialogporten.WebApi/Program.cs

@@ -75,6 +75,11 @@ static void BuildAndRun(string[] args)
         .ValidateFluently()
         .ValidateOnStart();
 
+    builder.Services.AddSingleton<IHostLifetime>(sp => new DelayedShutdownHostLifetime(
+            sp.GetRequiredService<IHostApplicationLifetime>(),
+            TimeSpan.FromSeconds(10)
+        ));
+
     var thisAssembly = Assembly.GetExecutingAssembly();
 
     builder.Services