Consider not using foreign keys. This could help with #338
GitHub does not use them github/gh-ost#331 (comment) allegedly
PlanetScape does not recommend them either https://planetscale.com/docs/vitess/operating-without-foreign-key-constraints

Considering that I added FKCs to help remove queries to clean-up some stuff this could be a step-back, but on the other hand, if it will improve overall performance and storage (some columns may not require index, but index is created due to FKC) - it may be worthwhile still, and could provide a bit more control from code level. And it would be a more explicit deletion, too.

This could also allow more data restoration logic. For example, I could delete a thread with all posts, but actually delete only the thread first, and then delete posts after a grace period. That way I can "recreate" a thread during that grace period without losing data.

This would require update to Cron, at least, from perspective of libraries.