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

tests/Unit/ArticlesTest.php

@@ -63,6 +63,17 @@ class ArticlesTest extends TestCase
         );
     }
 
+    #[Test]
+    public function uri_is_the_absolute_form_of_the_link(): void
+    {
+        $article = Article::create([
+            'title' => 'Test',
+            'main' => 'Test',
+        ]);
+
+        $this->assertEquals(config('app.url').$article->link, $article->uri);
+    }
+
     #[Test]
     public function date_scope_returns_expected_articles(): void
     {