DB Backend: mint `InsightsDB` interface

Created by: camdencheek

Have you wondered if that dbutil.DB you have is a handle to the frontend DB or the code insights DB? Well now you can wonder that less.

This PR mints an edb.InsightsDB type that plays a similar role to the database.DB type. This also replaces many instances where we use dbutil.DB for the code insights database with this named type. It's basically just a lightweight, typed wrapper around dbutil.DB that is also a ShareableStore.

This is one more step to getting rid of dbutil.DB.

Stacked on #36887 Part of https://github.com/sourcegraph/sourcegraph/issues/26113 in spirit.

Test plan

Should be semantics-preserving, but I'm depending heavily on tests here.