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/appsettings.json

@@ -5,5 +5,8 @@
       "Microsoft.AspNetCore": "Warning"
     }
   },
-  "AllowedHosts": "*"
+  "AllowedHosts": "*",
+  "BlogApi": {
+    "BaseUrl": "https://media.joshheaps.net"
+  }
 }