Start work on backfilling for compact indexes

[technicalpickles]

Description:

Backfill logic for #392

Tasks:

• backfill class to handle backfilling compact indexes
• CLI for backfilling
• database table for tracking needed backfills and their status
• update gemstash start (or more braodly) to check for pending backfills, and advise the user
• add a check to tell if compact indexes are available (ie has run the backfill)
• update PrivateSource to return 404 when compact indexes aren't available

I will abide by the code of conduct.