I'm searching for headless CMS on Yii2 to integrate it with my web app. The problem is I need only the one for editing database content. I don't need a typical CMS with caching, templates, pages and many other things.
Luya CMS supports headless use. From their site:
As LUYA is built upon the concept of REST APIs, providing headless access to the database content of LUYA is a no brainer. You can either access the APIs or the CMS admin APIs
If that is too complex to your need you can easily build REST API application based on your DB structure and support only functionality that you really need. I hope this helps.