Upload a new batch email file for processing

This endpoint allows users to upload and start processing a file containing multiple email addresses for batch processing. When a file is submitted, it is added to a queue for asynchronous processing and assigned a process_id. The file is marked as "QUEUED," and a message indicates that processing will begin soon. Each user has their own dedicated queue for tasks. The response includes the process_id and filename, along with the initial status of "PROCESSING QUEUED." Users can later check the status using the process_id.