Move synthetic dataset generation to Python backend

[SimoneMartino98]

Summary

• add utilities for building crops and synthesizing YOLO-format datasets in Python
• expose a /synthesize endpoint in the label tool server and update the UI to request backend generation
• cover the new generator with unit tests for crop extraction and archive creation

Testing

• pytest tests/vision/test_synthetic.py

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https://chatgpt.com/codex/tasks/task_e_68ee8ac5c870832db0772561f7140a4a