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>

database/factories/SiteFactory.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\Site;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+class SiteFactory extends Factory
+{
+    protected $model = Site::class;
+
+    public function definition(): array
+    {
+        return [
+            'name' => fake()->company(),
+            'url' => fake()->url(),
+            'description' => fake()->optional()->sentence(),
+        ];
+    }
+}