diff --git a/.env.example b/.env.example
new file mode 100644
index 0000000..616cf0f
--- /dev/null
+++ b/.env.example
@@ -0,0 +1,4 @@
+# Copy to .env and fill in. Never commit .env.
+GITEA_TOKEN=
+GITHUB_PAT=
+GITHUB_USER=
diff --git a/.gitignore b/.gitignore
index 36b13f1..ef1a1da 100644
--- a/.gitignore
+++ b/.gitignore
@@ -174,3 +174,6 @@ cython_debug/
 # PyPI configuration file
 .pypirc
 
+
+# Doxygen output
+docs/doxygen/
diff --git a/Doxyfile b/Doxyfile
new file mode 100644
index 0000000..c4684b8
--- /dev/null
+++ b/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
diff --git a/docs/plan.md b/docs/plan.md
new file mode 100644
index 0000000..07a9845
--- /dev/null
+++ b/docs/plan.md
@@ -0,0 +1,41 @@
+# Plan: Secret Auction Program (Day 9)
+
+## Context
+
+Udemy 100 Days of Code blind-auction CLI. The repository uses a Python `.gitignore`, an AGPL `LICENSE` and a git-ignored `.env` (tokens, never printed or committed). Python 3.13 or newer is required.
+
+## Working method
+
+Work on ONE milestone at a time. Each milestone is one branch (`milestone/NN-name`) and one pull request. When a milestone is finished, stop, verify it and review before starting the next. Milestones and issues are also tracked on the Gitea repository (via API, token from `.env`).
+
+## Milestone 1 - Project scaffold and repo metadata (`milestone/01-scaffold`)
+
+- [x] Create `src/secret_auction/__init__.py`, `tests/`, `docs/`
+- [x] `pyproject.toml`: `requires-python >=3.13`, src layout, no runtime deps, `dev` extra (`pytest`), pytest config, console script
+- [x] `.env.example`; `.env` and `.venv/` ignored; add `docs/doxygen/` to `.gitignore`
+- [x] `constants.py`
+- [x] `Doxyfile`
+- [x] Set repository description and topics (`python`, `udemy`, `100-days-of-code`, `blind-auction`, `beginner`) via API
+- [x] Create milestones and issues on the host (milestones M1-M4, issues #1-#12)
+- [x] Verify: create `.venv`, upgrade pip, `pip install -e ".[dev]"`
+- [x] Save plan and milestones in `docs/plan.md`
+
+## Milestone 2 - Core auction logic and env loader (`milestone/02-core-logic`)
+
+- [ ] `auction.py` (Doxygen comments): `add_bid`, `find_winner` (uses `max(bids, key=bids.get)`, `ValueError` on empty)
+- [ ] `env.py`: stdlib `.env` loader (no python-dotenv); does not override existing environment variables
+- [ ] `tests/test_auction.py`, `tests/test_env.py`
+- Verify: `pytest` passes
+
+## Milestone 3 - CLI program (`milestone/03-cli`)
+
+- [ ] `main.py`: prompt name and numeric bid (re-prompt on invalid), bids dict kept outside the loop, "other bidders? yes/no", clear screen with newlines between bidders, announce the winner
+- [ ] `tests/test_main.py` with monkeypatched `input`
+- Verify: `pytest`; scripted run `python -m secret_auction.main` with two bidders
+
+## Milestone 4 - Documentation (`milestone/04-docs`)
+
+- [ ] `README.md` from the template (Overview, Requirements, Setup incl. creating/activating `.venv` and `python -m pip install --upgrade pip`, Run, Tests, License AGPL, Links)
+- [ ] `docs/flowchart.md` (Mermaid plus link to the diagrams.net chart)
+- [ ] Generate Doxygen docs if `doxygen` is installed
+- Verify: README commands work verbatim; `.env` stays untracked
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..a585a93
--- /dev/null
+++ b/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"]
diff --git a/src/secret_auction/__init__.py b/src/secret_auction/__init__.py
new file mode 100644
index 0000000..954be24
--- /dev/null
+++ b/src/secret_auction/__init__.py
@@ -0,0 +1,5 @@
+"""! @file __init__.py
+@brief Secret Auction package: a blind auction command line program.
+"""
+
+__version__ = "0.1.0"
diff --git a/src/secret_auction/constants.py b/src/secret_auction/constants.py
new file mode 100644
index 0000000..f199f24
--- /dev/null
+++ b/src/secret_auction/constants.py
@@ -0,0 +1,39 @@
+"""! @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."
diff --git a/tests/.gitkeep b/tests/.gitkeep
new file mode 100644
index 0000000..e69de29