List Assignments

List role assignments scoped to one user.

• Omitting user_id defaults to the caller — no superuser needed.
• Passing user_id == self.id is the same as omitting it.
• Passing a different user_id requires superuser; otherwise 403.

Results are always filtered by the resolved user_id. Admins who need cross-user lookups make one call per user. Paginated via limit / offset (default 100, max 200).