fix: mail preview endpoint not loading and crashing on null data

Register web.php in bootstrap/app.php (was missing, so the route was
never loaded). Add null checks for all model queries with helpful error
messages instead of TypeErrors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/bootstrap/app.php

@@ -15,6 +15,7 @@ use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 return Application::configure(basePath: dirname(__DIR__))
     ->withRouting(
+        web: __DIR__.'/../routes/web.php',
         api: __DIR__.'/../routes/api.php',
         commands: __DIR__.'/../routes/console.php',
         health: '/up',