Personal opinion: Worrying about that is silly.

If someone wanted to write an app, they would. If they're buying yours it's because they don't have time/skills to do it themselves.

Take for example Microsoft sharepoint. Written in .net and SQL, all the source code is accessible. Does that mean I'm likely to spend hours pouring over the code to write a sharepoint clone for myself instead of using Sharepoint itself? I could, but it would be utterly stupid to do so.

It's highly unlikely that there's anything so revolutionary in your code/db design that it's worth someone reverse engineering and taking. If you feel differently, host the databases yourself.