Backups made within the same second overwrite each other #12

kicad_lib_manager/utils/backup.py:28 names backups <file>.backup.%Y%m%d_%H%M%S. Two runs of kilm setup, the env-var updates in utils/env_vars.py, or kilm import (PR #9) on the same file within one second get the same name, and shutil.copy2 overwrites the first backup with the already-modified file, so the original content is lost.

Suspected fix: add sub-second precision or a collision counter, keeping the names sortable for manage_backup_history. Found during review of PR #9; pre-existing on main.