Add Article::uri accessor for the absolute post URL

Follow the uri/link convention already used by Note, Bookmark, and
Place, rather than concatenating config('app.url') inline where the
absolute URL is needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017USyUg8PwuoDcHP8pv5xjy

app/Models/Article.php

@@ -93,6 +93,13 @@ class Article extends Model
         );
     }
 
+    protected function uri(): Attribute
+    {
+        return Attribute::get(
+            get: fn () => config('app.url').$this->link,
+        );
+    }
+
     /**
      * Scope a query to only include articles from a particular year/month.
      */