Feature: Add ONNX export pipeline and refactor CLI into subcommands

[fantasyRqg]

Summary

• Add soprano/onnx_export/ module with backbone and decoder ONNX export, end-to-end inference, and utility helpers
• Refactor CLI from a flat argument parser into tts, export, and test-e2e subcommands
• Maintain backwards compatibility: running soprano "text" without a subcommand still works as before
• Extract shared MODEL_ID constant into soprano/_constants.py
• Update backends and tts.py to use the shared constant
• Update .gitignore and pyproject.toml for ONNX export dependencies

Test plan

• uv run soprano tts "Hello world" works with the new subcommand
• uv run soprano "Hello world" still works (backwards compat)
• uv run soprano export exports ONNX models successfully
• uv run soprano test-e2e runs end-to-end ONNX inference