Make REST API pagination more consisent

note to self:

- some endpoints have namespaced pagination response (object, instead of array)
- pagination headers are not always present (not present if there is only one page), which makes it hard to recognize endpoints that do paginate

Start by inventorying the REST API endpoints and comparing their pagination response shapes and headers. The issue does not name files, tests, or entry points, so first establish the affected endpoints and the intended consistent behavior. Done means the pagination format and header presence are defined and consistent across the affected endpoints.

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