chore: import upstream snapshot with attribution

CONTRIBUTING.md

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+# Contributing to StemDeck
+
+Thanks for your interest in StemDeck - free, local stem separation for musicians. Contributions of
+all kinds are welcome, whether you write code or not.
+
+By participating you agree to follow our [Code of Conduct](CODE_OF_CONDUCT.md).
+
+## Ways to contribute
+
+- **Report a bug** - open a [Bug report](https://github.com/stemdeckapp/stemdeck/issues/new/choose).
+  Include your OS, the StemDeck version (Help icon -> About), and clear steps to reproduce.
+- **Suggest a feature** - open a [Feature request](https://github.com/stemdeckapp/stemdeck/issues/new/choose).
+- **Improve the docs** - fixes and clarifications to the README or these guides are always useful.
+- **Write code** - bug fixes and features. For anything large, please open an issue or a
+  [Discussion](https://github.com/stemdeckapp/stemdeck/discussions) first so we can agree on the
+  approach before you invest time.
+- **Found a security issue?** Do not open a public issue - see [SECURITY.md](SECURITY.md).
+
+## Project layout
+
+- `app/` - FastAPI backend (the audio pipeline, job registry, and HTTP API).
+- `static/` - the web UI: plain ES-module JavaScript, HTML, and CSS. No build step, no bundler.
+- `desktop/` - the Tauri v2 desktop shell (Rust) that wraps the backend + UI.
+- `scripts/` - packaging scripts (macOS app/dmg, Windows portable, runtime pack).
+- `tests/` - the pytest suite for the backend.
+
+## Development setup
+
+You need [`uv`](https://docs.astral.sh/uv/) and `ffmpeg` on your PATH.
+
+```bash
+uv sync --python 3.12        # install the Python environment
+./run.sh start               # start the backend at http://localhost:8000
+```
+
+Other helpers:
+
+```bash
+./run.sh restart             # restart after backend changes
+./run.sh stop                # stop the server
+./run.sh status              # is it running?
+```
+
+Open `http://localhost:8000` in your browser to use the app. Frontend changes are picked up on
+reload (no build step).
+
+## Before you open a pull request
+
+Please run the same checks CI runs:
+
+```bash
+uv run ruff check app/ tests/        # lint
+uv run ruff format --check app/ tests/   # formatting
+node --check static/js/<changed>.js  # syntax-check any JS you touched
+uv run pytest tests/ -q              # backend tests
+```
+
+For Rust changes in `desktop/`, also run `cargo fmt --check` and `cargo clippy -- -D warnings`.
+
+## Pull request guidelines
+
+- Branch off `main`.
+- Use [Conventional Commits](https://www.conventionalcommits.org/) for messages
+  (`feat:`, `fix:`, `docs:`, `refactor:`, `test:`, `chore:`, `ci:`).
+- Open the PR as a **draft** until it is ready for review.
+- Keep PRs focused - one logical change per PR makes review faster.
+- Describe what changed and why, and how you tested it.
+
+## Tests
+
+New API endpoints and pipeline stages should come with tests under `tests/`. The suite uses
+`pytest` with `httpx.AsyncClient`; see the existing `tests/test_*.py` files for patterns.
+
+## License
+
+StemDeck is licensed under the [Apache License 2.0](LICENSE). By contributing, you agree that your
+contributions are licensed under the same terms.
+
+## Questions
+
+Open a [Discussion](https://github.com/stemdeckapp/stemdeck/discussions) or join the
+[Discord](https://discord.gg/2MVsWqaPRe). Thanks for helping make StemDeck better.