⚡ Replace Thread.Sleep with Task.Delay in SettingsService

[spelech]

Summary

Addresses failed Jules session \13362444024584746614.

💡 What

• Replaced blocking \Thread.Sleep(50)\ calls in file I/O retry loops with non-blocking \�wait Task.Delay(50, cancellationToken).
• Added \LoadSettingsAsync(CancellationToken cancellationToken = default)\ and \SaveSettingsAsync(AppSettings settings, CancellationToken cancellationToken = default)\ to \ISettingsService\ with default interface implementations for 100% backward compatibility with all existing callers and test mocks.
• Replaced \lock (SettingsLock)\ with an asynchronous-safe \SemaphoreSlim\ in \SettingsService\ to allow proper non-blocking synchronization across both async and sync callers.
• Updated /api/settings\ GET and POST endpoints in \EngineEndpoints.cs\ to utilize async settings operations.
• Added comprehensive unit tests in \AppSettingsTests\ verifying async load/save roundtrip and cache sharing between sync and async calls.

🎯 Why

• Blocking threads with \Thread.Sleep\ inside locks starves the ASP.NET Core thread pool during transient retry delays.
• Asynchronous file I/O and \Task.Delay\ prevent thread pool exhaustion.