Create broadcast flow

A BroadcastFlow represents a bulk patient outreach campaign that sends the same message via SMS, email, or voice to a batch of patients, either from an uploaded contact file, an SFTP feed, or a filtered set of appointments. It tracks the full lifecycle of that campaign including upload and column mapping, optional approval review, scheduling, batched message sending, and per-row success and failure statistics, and can be organized into parent and child flows for large batches, commonly used for patient recall and other mass communications.