feat(bookstack) : add BookStackApiException

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Exception/BookStackApiException.php

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exception;
+
+use RuntimeException;
+use Throwable;
+
+final class BookStackApiException extends RuntimeException
+{
+    public function __construct(string $message, int $code = 0, ?Throwable $previous = null)
+    {
+        parent::__construct($message, $code, $previous);
+    }
+}