[Feature Request] A hook to transform a field before it's saved and after it's read (for field-level encryption)

Description Paperless is the privacy-friendly choice, which is exactly why I'd like to go one step further: have some of the text stored encrypted at rest. A document's title, its OCR content, the correspondent, my notes. Right now that all sits in the database as plain text, so a stolen database file, an old backup, or the search index on disk gives someone the whole archive to read. I can handle the encryption itself. What I can't do without forking is hook the values on the way in and out, because that means editing the model and serializer classes directly. I'd like an opt-in way to say "run these fields through a transform on save and on read", without touching the models. Something like registering, at startup, which fields a transform owns and what it does (encrypt on write, decrypt on read for the current user). Under the hood it's the pre_save signal Paperless already uses on the write side, and the DRF serializers on the read side. W…