Add support for system users and a system user for prebuilds. System users are used to perform automated tasks that cannot belong to any specific user, such as managing prebuilt workspaces.

Prebuilt workspaces, like all workspaces, need to be owned by someone according to our data model. They should not be owned by a regular user until they are claimed by that user. As such, we need a system user.

We should also prevent modification of system users ensure that operations like user suspensions, deletions, role changes, or profile updates can't accidentally break system functionality that depend on these accounts.

System users should not be visible by default.