> Would Hister be suitable for this?

The indexer it uses (Bleve) can handle millions of records according to their docs, but sure that Hister would be the best choice for this task. I'd probably use Meilisearch (https://www.meilisearch.com/).

> Can it index mbox files?

Not yet.

> Does it have a search API so I can build an MCP server?

It has both search API and MCP server endpoints.