Adds test_doclist_profile.py with 8 profiling tests covering the
/api/documents/ list path (ORM ordering, page sizes, single-doc detail,
cProfile) and _get_selection_data_for_queryset in isolation and via API.
Also registers the 'profiling' pytest marker in pyproject.toml.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>