fix: add path traversal validation to datapoints endpoints

[q1uf3ng]

Summary

_save_datapoints and _load_datapoints in app.py pass user-supplied file paths directly to open() without validation. A path like ../../etc/passwd reads or writes arbitrary files on the server.

Fix

Add _validate_data_path() that resolves paths with os.path.realpath() and rejects anything outside the configured data_dir.

Reproduction

# With LIT server running:
requests.post('http://localhost:5432/save_datapoints', json={
    'dataset_name': 'sst_dev', 'path': '../../tmp/stolen',
    'inputs': [{'text': 'test'}]
})

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[q1uf3ng]

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