Study and credit online_migrations

[kaaveland]

This looks like an excellent resource, and I can see some new patterns in the README: https://github.com/fatkodima/online_migrations

[kaaveland]

Didn't know you could drop indexes concurrently 💡

We can warn about adding short IDs as primary keys int4. 🤔

I feel like we're already handling adding multiple foreign keys in one migration ok, since we allow adding them as NOT VALID and ask to validate them separately.

It suggests dropping foreign keys before dropping the referencing table, will look into that.

Didn't know that postgres allows the "Mismatched reference column types" case, will have to look into that too.

All these seem manageable/easy to add to the tracer.