Create an equipment downtime. `startDateTime` and `endDateTime` are UTC; the end cannot be earlier than the start. Each `departments` entry must be a department that exists, listed once, and each of its `equipment` entries must be equipment in that department, listed once for it. An empty or omitted `equipment` list means the whole department is down. Departments and equipment are saved in request order, and the server fills `departmentName` and `equipmentName` from the records, so client values are ignored.