Forgejo hack: How to set a starting issue and pull request number

I'm currently working on migrating my open source projects from GitHub to a self-hosted Forgejo instance. As part of this effort I often end up looking through the Forgejo source code to figure out if there are hidden ways to configure certain things to my liking when I can't do it on the administration UI. I thought I'd start putting my discoveries in writing here, in case they can help others. So here goes the first one.

One of the aspects of the migration that is tricky is how to transition issues and pull requests. What makes the most sense to me is to only use Forgejo to track issues and pull requests going forward, leaving all the issues and pull requests created up to the migration point on GitHub. Of course whether this is a good or bad idea is debatable, but considering all the options I have decided that this is the solution that is going to inflict the least pain on me.

The one problem with this approach is that I would end up having duplicate issue numbers, because Forgejo would start creating issues and pull requests all the way back from #1, and all those low numbers have been used on the GitHub side. So I wanted to hack my Forgejo instance so that issues start from, say, 10000. That way when anyone references an issue by its number I would know that numbers below 10000 are on GitHub and only those above are on my own instance.