Push the corpus into SQLite, it has built-in FTS engines[^1]. Then serve it with anything. Unfortunately this needs server side code, but like 30 lines of PHP.
You can do SQLite in the browser but it’ll have to download the entire dB file instead of only opening the pages it needs (because the naive JS port can’t convert page requests to range requests).
It should be possible to support loading only the required pages on the browser with SQLite compiled to WASM along with a custom VFS implementation. Here’s a project[1] which does something similar (selectively load the SQLite DB on demand), albeit with the SQLite file in a torrent.