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>

app/Models/Site.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+class Site extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'name',
+        'url',
+        'description',
+    ];
+}