feat: add CLI options, shared path util, requirements (by Basuki)

[basukinath]

Summary of changes:

• Added argparse-based CLI to scanner.py (--host, --low, --high, --threads, --no-banner)
• Centralized duplicate path logic into path_utils.py
• Added requirements.txt declaring Flask
• Improved resilience if config.json missing or invalid (defaults to 1–1024, 8 threads)
• Added cross-version (Py2/Py3) compatibility shims

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