codeintel: Add skeleton upload expirer job

Created by: efritz

This PR adds a skeleton (no-op) worker job that will (when implemented) compare upload records against data retention policies and decide if they should be marked as expired.

Later PRs will add additional jobs, including the expiredUploadDeleter, which will take expired uploads with no references and mark them as deleted and unavailable to answer code intelligence queries.

Adding the fanfare of a new background task separately so that future PRs can be focused as there are some tricky bits that I'll need your full, 100%, Lucy-type brainpower for.