Launch batch campaign

Full URL: POST https://api.kupe.in/api/v1/batch-call/start/{batch_id}

Starts dialing: claims up to batch_size not_started rows, initiates outbound calls, and continues in waves as calls complete. Sets batch status to in_progress; when all rows finish, status becomes completed.

Returns immediately while dialing runs in the background. Poll GET .../{batch_id}/status or GET .../{batch_id}/data for per-number progress.