Lint a rules file

Format

Auto-detect picks Cursor rule (.mdc) when the frontmatter has description, globs, or alwaysApply. The format only changes whether R015 and R016 run.

Rules file content

# Python FastAPI service

Python 3.12, FastAPI, SQLAlchemy 2.0 (async), PostgreSQL, Alembic. Dependencies and commands run through uv.

## Commands
- Install: `uv sync`
- Run: `uv run fastapi dev src/<app>/main.py`
- Test: `uv run pytest`
- Lint: `uv run ruff check .`
- Format: `uv run ruff format .`
- Type check: `uv run mypy src`
- New migration: `uv run alembic revision --autogenerate -m "<summary>"`, then read the generated file before committing it.

## Layout
- `src/<app>/api/`: routers, one module per resource.
- `src/<app>/services/`: business logic. No FastAPI imports.
- `src/<app>/db/`: models, session, repositories.
- `tests/` mirrors `src/<app>/`.

## API
- Request and response bodies are Pydantic v2 models in `src/<app>/schemas/`. ORM models are never returned from a route.
- Routes declare `response_model` and an explicit `status_code`.
- `HTTPException` is raised only in routers. Services raise domain errors from `<app>.errors`, and `api/errors.py` maps them to status codes.
- Database sessions come from the `get_session` dependency. Services do not open sessions.

## Money
- Amounts are integers in minor units (cents). No floats for money.
- Every amount is stored with an ISO 4217 currency code next to it.

## Code style
- Every function signature has type hints. `mypy --strict` passes for `src/`.
- Ruff rules live in `pyproject.toml`. A `# noqa` comment always names the rule code.
- Timestamps use `datetime.now(tz=UTC)`. Naive datetimes are a bug.

## Tests
- pytest with `pytest-asyncio`. Async tests use the `async_client` fixture from `tests/conftest.py`.
- Database tests run against the Postgres container started by `docker compose up -d db`.
- A bug fix starts with a failing test that reproduces the bug.

## Done
- `ruff check`, `ruff format --check`, `mypy`, and `pytest` all pass.

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Rules

Rules to run R001 Rationale prose R002 Vague verbs R003 Contradictory modals R004 File length R005 Duplicate lines R007 Hedged instructions R008 Emphasis inflation R009 Secrets in rules file R010 Persona preamble R011 Empty sections R012 Unclosed code fence R013 Prose paragraph R014 Unverifiable cross-reference R015 Frontmatter fields R016 Manual Cursor rule

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