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

Doxyfile

@@ -0,0 +1,12 @@
+PROJECT_NAME           = "Secret Auction"
+PROJECT_BRIEF          = "A blind auction command line program"
+OUTPUT_DIRECTORY       = docs/doxygen
+INPUT                  = src README.md
+FILE_PATTERNS          = *.py *.md
+RECURSIVE              = YES
+OPTIMIZE_OUTPUT_JAVA   = YES
+EXTRACT_ALL            = YES
+USE_MDFILE_AS_MAINPAGE = README.md
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES