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/Pages/Blog/Post.cshtml.cs

@@ -0,0 +1,28 @@
+using JoshHeaps.Net.Models;
+using JoshHeaps.Net.Services.Interfaces;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace JoshHeaps.Net.Pages.Blog;
+
+public class PostModel : PageModel
+{
+    private readonly IBlogService _blogService;
+
+    public BlogPost? Post { get; set; }
+
+    public PostModel(IBlogService blogService)
+    {
+        _blogService = blogService;
+    }
+
+    public async Task<IActionResult> OnGetAsync(string slug)
+    {
+        Post = await _blogService.GetPostBySlugAsync(slug);
+
+        if (Post is null)
+            return NotFound();
+
+        return Page();
+    }
+}