fix: replace atexit with BackgroundTask for temp zip cleanup

[hobostay]

Summary

• Replace atexit.register(cleanup_zip) with Starlette's BackgroundTask for cleaning up temporary zip files after session downloads
• atexit handlers only run at process shutdown, so every download accumulates a temp zip file on disk until the server restarts

Bug Details

In server/routes/sessions.py, after creating a temporary zip archive for download, the cleanup is registered via atexit:

def cleanup_zip():
    if zip_path.exists():
        zip_path.unlink()

atexit.register(cleanup_zip)

Problem: atexit handlers only run when the Python process exits. In a long-running server, this means:

1. Each download creates a new temp zip file
2. None of them are cleaned up until the server restarts
3. Under high usage, temp files accumulate and consume disk space
4. Each atexit.register call also adds to a growing list of callbacks

Fix: Use Starlette's BackgroundTask which runs after the HTTP response is fully sent.

Test plan

• Download a session and verify the zip file is created and served correctly
• Verify the temp zip file is deleted after the download completes

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