Shorter, unified retention for build archives and run artifacts, plus legacy artifact cleanup

Build archive and run artifact downloads now expire after 30 days across every plan, and the server does a more thorough cleanup pass in the background. The shipped change unifies the retention window so build and run binaries are kept for the same period regardless of tier, instead of staying around based on plan. It also adds a daily hosted worker that scans the backing storage bucket for legacy build objects the existing metadata-driven cleanup could not see, so expired legacy artifacts (including those under orphaned account prefixes) are removed too. Result: less stale storage holding old builds, and the cutoff for downloadable build and run files is a predictable 30 days.