Keep frequently used packages up to date in the background #632

mirror.schedules keeps container image tags current, but for packages the cache only learns about a new release when a client asks for it. On a home or CI proxy that is often offline-first, the first install after a release then waits on the upstream, or fails while it is down.

Idea: an opt-in background job that refreshes what is used most.

mirror:
  refresh:
    interval: 24h
    top: 50          # most downloaded packages per ecosystem, by recent hits
    since: 14d

Each run, for each of those packages:

• revalidate the metadata (packument, Composer p2 file, PyPI simple page, maven-metadata.xml, …)
• if there is a newer version on a major line already cached, mirror it, with the newest version cooldown allows rather than the newest published
• for images, re-mirror the cached tags the way schedules do

Open questions:

• Which files: npm, Composer, Cargo and gems have one archive per version. PyPI and conda have many; mirroring only the variants clients fetched before (same wheel tags, same subdir) seems right.
• Major lines: newest per cached major line, or only the overall latest (Add --latest to mirror only the newest version #631 adds --latest for the latter)?
• Storage: new versions add up; max_size and retention evict old ones, but a cap on files or bytes per run may be needed.
• Debian/Alpine/RPM are index-driven and probably out of scope at first.
• With several instances on one database, run it once per interval like schedules (job_runs).

Happy to work on it if this fits the project.