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

src/secret_auction/__init__.py

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+"""! @file __init__.py
+@brief Secret Auction package: a blind auction command line program.
+"""
+
+__version__ = "0.1.0"

src/secret_auction/constants.py

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+"""! @file constants.py
+@brief Constants used by the Secret Auction program.
+"""
+
+## Number of blank lines printed to hide previous bidders' entries.
+CLEAR_SCREEN_LINES = 100
+
+## Answers accepted as "yes" when asking for more bidders.
+YES_ANSWERS = ("yes", "y")
+
+## Answers accepted as "no" when asking for more bidders.
+NO_ANSWERS = ("no", "n")
+
+## Path of the file holding environment variables (tokens).
+ENV_FILE = ".env"
+
+## Name of the environment variable holding the Gitea token.
+GITEA_TOKEN_ENV_VAR = "GITEA_TOKEN"
+
+## Prompt asking for the bidder's name.
+PROMPT_NAME = "What is your name? "
+
+## Prompt asking for the bid amount.
+PROMPT_BID = "What is your bid? $"
+
+## Prompt asking whether more bidders exist.
+PROMPT_MORE_BIDDERS = "Are there any other bidders? Type 'yes' or 'no'. "
+
+## Message shown when the bid is not a valid number.
+MSG_INVALID_BID = "Please enter a valid, non-negative number."
+
+## Message shown when the yes/no answer is not understood.
+MSG_INVALID_ANSWER = "Please type 'yes' or 'no'."
+
+## Template for the winner announcement.
+MSG_WINNER = "The winner is {name} with a bid of ${amount:.2f}."
+
+## Welcome banner.
+MSG_WELCOME = "Welcome to the secret auction program."