Rename slug to id?

Some primary keys use slug and others use id, which makes writing client code difficult, since you have to know in advanced whether an object is using slug or id. Since it doesn't matter whether the pk is a slug or not as it's autogenerated from the name anyway, it seems better to rename everything to ID.

No files, tests, or entry points are named. Start by identifying every model and client-facing API object that uses slug or id, then determine the migration and compatibility scope. Done means primary-key naming is consistent without breaking existing client code.

Written by the indexing model from the issue text.