You can modify the .htaccess file of your wordpress installation to pass through all requests to your /app/ folder without going through wordpress as follows:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
# Let all go through to app
RewriteCond %{REQUEST_URI} "/app/"
RewriteRule (.*) $1 [L]
# Normal Wordpress rewrites
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress