Target block size in bytes; defaults to BACKUP_UPLOAD_WRITE_CHUNK_TARGET.
Appends a source chunk and returns full target-sized blocks ready to write.
Bytes from the upstream video stream.
Chunks of up to the configured target size.
Drains any bytes smaller than the target chunk size at end of stream.
Final partial chunk, if any.
Batches small ReadableStream chunks into fewer network writes for better throughput on large files.