worker: Migrate codeintel auto indexing jobs

Created by: efritz

This PR moves the background jobs that queue auto-indexing jobs from the frontend. Fixes https://github.com/sourcegraph/sourcegraph/issues/21763. Last fix for https://github.com/sourcegraph/sourcegraph/issues/18253.

This is the first set of jobs being put into the worker, and docs and additional observability efforts will follow shortly after we ensure that this runs in dogfood and cloud clusters properly.