fix: add scipy as explicit runtime dependency

[mattdawkins]

Summary

• Add scipy to requirements/runtime.txt and setup.cfg install_requires
• batbot/spectrogram/__init__.py imports directly from scipy.stats (median_abs_deviation, skew, norm) but scipy was not listed as an explicit dependency
• Previously only available transitively through librosa and scikit-image, which is fragile and could break if those packages change their dependency trees

Test plan

• Verify pip install . in a clean virtual environment pulls in scipy
• Run existing tests to confirm no regressions

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