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/migrations/2026_07_18_114506_create_sites_table.php

@@ -0,0 +1,24 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('sites', function (Blueprint $table) {
+            $table->id();
+            $table->string('name');
+            $table->string('url');
+            $table->text('description')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('sites');
+    }
+};