feat: library audit --fix

An audit that only reports leaves the same chore every time.

--fix rebuilds the library copies that are missing or short, and restores a missing cover.jpg by copying the archive's rather than re-encoding the album, which would be wasted work for one file.

It never writes to the archive. The masters are the one thing a rebuild cannot replace, so a missing or empty archive directory is reported and left strictly alone, and so is an album in the archive that no record names.

Without --fix the report now says that --fix exists, and both modes count what was repaired in --json.