Replace file-based blog with API-backed service

BlogService now fetches from Media.JoshHeaps.Net API via HttpClient
with in-memory caching (5-min TTL) and stale cache fallback.
Removed Markdig/YamlDotNet dependencies. All page models now async.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

JoshHeaps.Net/Program.cs

@@ -12,6 +12,13 @@ builder.Services.AddControllers();
 
 builder.Services.AddSignalR();
 
+builder.Services.AddHttpClient("BlogApi", client =>
+{
+    var baseUrl = configuration["BlogApi:BaseUrl"] ?? "https://media.joshheaps.net";
+    client.BaseAddress = new Uri(baseUrl);
+    client.Timeout = TimeSpan.FromSeconds(10);
+});
+builder.Services.AddSingleton<IBlogService, BlogService>();
 builder.Services.AddSingleton<IChessService, ChessService>();
 builder.Services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();