add file metadata as separate columns instead of embedding timestamp in file

[lun-4]

instead, attach more metadata to the files table, instead of just "timestamp and local path", possibly add things like mtime, ctime, maybe inode number, that would let us possibly have more heuristics about the file, while also not having to do the large id migration, which, to be honest, is a lot of code (which has a bug, since i used ctime instead of mtime, so all files had incorrect timestamps when ordered in szurubooru. if anyone wants the fix script I can commit it, it's just not my proudest fix)