Feature request: Modify subject line of downloaded emails [for restore to group]

First, thanks for a great project.

I'm using GYB to download emails to my local, for restore to a google group for long term storage.

I'd like to prefix the subject line of all the emails, so I can tell who it was originally sent to

I had an llm make this script for me: https://gist.github.com/YesThatAllen/9ff75735a985955a7d6608c23dd2e108

When I do run this on the local backup before having GYB upload to the group, this worked great.

I thought I'd submit this to the team for consideration, or so people could use the idea even if there's not a great place to put it in to GYB directly.

Start by reviewing the linked gist and the GYB download and restore workflow to identify where subject lines can be transformed. Clarify the desired prefix configuration and whether it applies during download, upload, or both. Done should allow restored messages to show their original recipient in the subject without changing unrelated message content.

Written by the indexing model from the issue text.