Support Large Files

[philomath213]

Summary

1. File uploads previously read the entire file into memory with f.read() before POSTing, causing socket.send() to fail on large files.
2. Now passes the open file handle directly to requests, which streams the body in chunks.
3. HMAC-SHA256 signing is preserved by hashing the file incrementally (64 KB chunks) before seeking back to 0.

Test plan

• pytest passes
• Upload a large ZIP file (bhcli upload large.zip) and confirm it completes without a socket/buffer error