I actually used this recently in a small personal project, it's pretty good. It's not like Elastic or Solr, more like Lucene - which may very well be good enough for your use case. The index structures are stored in BoltDB (which stores in a flat file.)

There are some issues, though. For example I think it's currently not possible to use the built-in query language to search Boolean values. So you might run into some minor issues even in smaller projects.