keep track of copied and generated resources in db

[xworld21]

Postprocessing could track which files are being written to the site directory, with relevant metadata (mime type, parent document?). This would make manifest generation more reliable: rather than scanning the folder and re-guessing the mime types, all the information would already be available in memory.

(I first mentioned this in https://github.com/brucemiller/LaTeXML/pull/1951#discussion_r962146624)