Access: System, Admin, and ClientAdmin users only.
Request format is multipart/form-data with file field file. Import is create-only (existing records are not updated/deleted).

Row processing is independent and partial: invalid rows are marked and processing continues until 5 consecutive row errors are reached.
Missing `End Date` defaults to `Start Date`. Missing `Country` defaults to `US`.
On success returns `200` with JSON summary. On row-level validation errors returns `422` with JSON summary and annotated XLSX (multipart/mixed). Rows already marked as `Success` and locked are skipped on re-import.