feat: scaffold project structure and add configuration files

- Add src/secret_auction package with __init__.py and constants.py
- Configure pyproject.toml with setuptools, pytest, and console script
- Add Doxyfile and .env.example for documentation and environment setup
- Update .gitignore to exclude Doxygen output directory
- Include tests directory placeholder and docs/plan.md for milestone
  tracking

pyproject.toml

@@ -0,0 +1,26 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "secret-auction"
+version = "0.1.0"
+description = "A beginner-friendly blind auction program (Udemy 100 Days of Code, Day 9)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = "AGPL-3.0-or-later"
+license-files = ["LICENSE"]
+dependencies = []
+
+[project.optional-dependencies]
+dev = ["pytest>=8"]
+
+[project.scripts]
+secret-auction = "secret_auction.main:main"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src"]