Both fields have existed on the model and been shown in the File
Manager for some time, but nothing ever wrote to them — every file in
every library appeared to have never been printed.
Now on_print_complete's queue-status update path calls a small
_bump_library_file_usage_if_completed() helper that increments
print_count and stamps last_printed_at on the source library file
whenever a queued print completes successfully. Failed, cancelled and
user-aborted prints are intentionally skipped so the fields represent
successful usage rather than attempt count.
Unblocks sorting the File Manager by last-printed date and is a
prerequisite for the scheduled-purge feature requested in #1008,
which is held until we see whether manual sort+bulk-delete covers the
use case.