A ServiceStack Service is the implementation of a Web Service or HTTP API (it does support other use-cases but I’ digress). MVC Controllers is the C in the MVC pattern typically used to display server generated HTML UIs.
Services have no correlation with a DB Entity, the Service should just be designed to accomplish what it needs to do. I wouldn’t try imposing a forced API design around rules like DB entities, you should have the freedom to accomplish each API as you see fit.