The MySQL server is not exposed. Are you OK to be able to use the mysql CLI?
You can access the database either
a) Web terminal -> Click the mysql button on the top -> Press enter. This drops you into mysql shell. You can then do 'show tables' etc.
b) Install Cloudron CLI (npm install -g cloudron) on your laptop/PC. Then:
cloudron exec --app ninja.example.com -- bash -c 'mysql --user=${CLOUDRON_MYSQL_USERNAME} --password=${CLOUDRON_MYSQL_PASSWORD} --host=${CLOUDRON_MYSQL_HOST} ${CLOUDRON_MYSQL_DATABASE} -e "SHOW TABLES"';
(If you remove the -e "SHOW TABLES" part, you will get a shell).