Migrate packaging to pyproject.toml (0.4.1)

[valbertovc]

Summary

• Replace setup.py with PEP 621 pyproject.toml using Hatchling as the build backend
• Replace the standalone dokkupycli script with a [project.scripts] console entry point
• Apply PEP 639 license metadata (license + license-files) and remove deprecated License :: trove classifier
• Update README install/dependency/CLI examples and remove redundant requirements.txt
• Bump version to 0.4.1

Test plan

• python -m build produces sdist and wheel
• twine check dist/* passes
• pip install dist/*.whl and dokkupycli --help works

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