PocketBase

PocketBase is an open-source backend solution provided in a single file, offering features such as a realtime database, authentication, file storage, and an admin dashboard. It can be used as a standalone app or extended as a framework via Go and JavaScript hooks. PocketBase simplifies backend development by providing essential components out-of-the-box.

PocketBase is a strong candidate because it offers a backend solution with a realtime database and authentication, aligning with the 'API & Database Integrations' and 'Access Control & Security' features. The admin dashboard and file storage capabilities further support its classification as a tool for building internal applications, although it may not be as extensive as Retool in terms of drag-and-drop UI building.