Export every active expenditure item for the claim as a CSV file.

Admin-only. Returns the reviewer-focused column set defined by _EXPORT_CSV_COLUMNS — one row per AURDExpenditureItem. Ignores any on-screen filter (tab, date range, eligibility) so the output is a stable snapshot of the imported journal items reviewers can reconcile against source records.

Synthesised salary rows (from _fetch_salary_rows) are intentionally not included — they're a UI projection over aurd_employee_expenses, not transactions on this table.

Auth is intentionally a pair of dependencies: get_admin_user checks the real session owner (so an admin impersonating a non-admin still passes, and a non-admin can never reach this regardless of impersonation), and get_released_salary_visible_claim_for_user checks claim access on the active identity. Don't collapse them.