Set English Piper voice default

[Duvet05]

Summary

• set the default English Piper model path to /srv/piper/voices/en_US-lessac-medium.onnx
• document that CPU deployments should include both Spanish and English Piper models
• add a test for the English voice default

Verification

• .venv/bin/pytest tests/test_local_providers.py tests/test_session.py
• .venv/bin/python -m py_compile src/config.py src/session.py src/providers.py

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