feat: add Site model, migration, and factory

Implements Task 2: creates the Site model with fillable name, url, and
description fields; the corresponding database migration; and a factory
for generating test data. Includes TDD tests verifying model creation
with and without optional description.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/Feature/SiteModelTest.php

@@ -0,0 +1,25 @@
+<?php
+
+use App\Models\Site;
+
+test('a site can be created with name, url, and optional description', function () {
+    $site = Site::create([
+        'name' => 'My Blog',
+        'url' => 'https://example.com',
+        'description' => 'A blog about things',
+    ]);
+
+    expect($site->fresh())
+        ->name->toBe('My Blog')
+        ->url->toBe('https://example.com')
+        ->description->toBe('A blog about things');
+});
+
+test('a site can be created without a description', function () {
+    $site = Site::create([
+        'name' => 'My Blog',
+        'url' => 'https://example.com',
+    ]);
+
+    expect($site->fresh()->description)->toBeNull();
+});