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CONTRIBUTING.md

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+# Contributing to Oikos
+
+Thanks for your interest in contributing. Oikos is a small, opinionated project — this guide covers the constraints and conventions you need to know before submitting code.
+
+## Ground Rules
+
+**Oikos has a hard "no frameworks, no build tools" constraint.** This is not a temporary limitation — it's an architectural decision. Specifically:
+
+- No React, Vue, Svelte, Angular, or any frontend framework
+- No Webpack, Vite, Rollup, esbuild, or any bundler
+- No TypeScript (plain JavaScript with ES modules)
+- No CSS libraries (Tailwind, Bootstrap, etc.)
+- No external frontend dependencies except Lucide Icons (self-hosted SVG sprite)
+
+If your contribution requires adding a frontend dependency, it will not be merged. Backend dependencies are evaluated case-by-case but should remain minimal.
+
+## Getting Started
+
+```bash
+git clone https://github.com/ulsklyc/oikos.git
+cd oikos
+npm install
+cp .env.example .env
+# Set SESSION_SECRET — skip DB_ENCRYPTION_KEY for local dev
+npm run dev
+```
+
+Run tests before submitting:
+
+```bash
+npm test    # Requires Node >=22 (uses --experimental-sqlite)
+```
+
+## Code Conventions
+
+### General
+
+- ES modules everywhere (`import`/`export`, never `require`)
+- Semicolons: **yes**
+- Header comment in every file: purpose, module, dependencies
+- `try/catch` in every route handler — no unhandled promise rejections
+- No `eval()`, no `innerHTML` with user input — use `textContent` or DOM API
+
+### Frontend
+
+- Web Component prefix: `oikos-` (one component per file)
+- All UI text in **German** (the app targets German-speaking families)
+- Date format: `DD.MM.YYYY` — Time format: `HH:MM` (24h)
+- Pages are ES modules in `public/pages/` that export a `render()` function
+- CSS uses design tokens from `public/styles/tokens.css` — don't hardcode colors or radii
+
+### Backend
+
+- API routes live in `server/routes/`, one file per module
+- API responses: `{ data: ... }` on success, `{ error: string, code: number }` on failure
+- Database migrations go in the `migrations` array in `server/db.js` — **append only, never modify existing entries**
+- Every table: `id INTEGER PRIMARY KEY`, `created_at TEXT`, `updated_at TEXT`
+
+### Testing
+
+- Tests use the Node.js built-in test runner with in-memory SQLite (`--experimental-sqlite`)
+- One test file per module in `tests/`
+- No running server needed — tests import route handlers directly
+
+## Submitting Changes
+
+1. Fork the repo and create a branch from `main`
+2. Make your changes following the conventions above
+3. Add or update tests if applicable
+4. Run `npm test` and make sure all tests pass
+5. Open a pull request with a clear description of what and why
+
+Keep PRs focused. One feature or fix per PR. Large refactors should be discussed in an issue first.
+
+## Reporting Bugs
+
+[Open an issue](https://github.com/ulsklyc/oikos/issues/new) with:
+
+- What you expected to happen
+- What actually happened
+- Steps to reproduce
+- Environment (browser, OS, Docker version if relevant)
+
+## Feature Requests
+
+[Open an issue](https://github.com/ulsklyc/oikos/issues/new) describing the use case. Explain the problem before proposing a solution — there might be a simpler approach that fits the existing architecture.
+
+Features that conflict with the project's constraints (see above) or significantly expand scope will likely be declined. When in doubt, ask first.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](LICENSE).