refactor: rewrite restoreProtectedPlugins to use file copy instead of git

- Dockerfile: backup plugins/ to /opt/default-plugins/ at build time
- Replace exec/git-based restore with pure PHP File::copyDirectory()
- Only restore plugins defined in Plugin::PROTECTED_PLUGINS
- Delete target directory before copy to prevent stale files
- Remove function_exists('exec') guard (no longer needed)

Dockerfile

@@ -32,6 +32,7 @@ COPY .docker/supervisor/supervisord.conf /etc/supervisor/conf.d/supervisord.conf
 
 RUN composer install --no-cache --no-dev \
     && php artisan storage:link \
+    && cp -r plugins/ /opt/default-plugins/ \
     && chown -R www:www /www \
     && chmod -R 775 /www \
     && mkdir -p /data \