feat: add read-only Sites API

Add GET /api/sites endpoint for listing sites with optional name filter,
and GET /api/sites/{site} for showing individual sites. Includes API
resource, controller, routes registration, and comprehensive tests.

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

routes/api.php

@@ -0,0 +1,7 @@
+<?php
+
+use App\Http\Controllers\Api\SiteController;
+use Illuminate\Support\Facades\Route;
+
+Route::get('/sites', [SiteController::class, 'index']);
+Route::get('/sites/{site}', [SiteController::class, 'show']);