chore: restructure into monorepo and add open-source documentation

- Rename helix-server -> server and helix-web -> web
- Remove helix-api (bruno collection and docs site)
- Merge per-module .gitignore files into the root .gitignore
- Add README, AGENTS, CONTRIBUTING and MIT LICENCE
- Add full-stack docker-compose (postgres, redis, server, web via Caddy)
  with web Dockerfile, Caddyfile and Compose server config
- Keep the Gradle wrapper jar tracked after the .gitignore merge
- Copy gradle.properties into the server Docker build image

CONTRIBUTING.md

@@ -0,0 +1,70 @@
+# Contributing to Helix
+
+Thanks for your interest in contributing! Helix is open source under the
+[MIT licence](LICENCE). This guide explains how to set up your environment,
+make changes and get them merged.
+
+## Getting started
+
+1. Fork the repository and clone your fork.
+2. Install the prerequisites:
+   - Backend: JDK 21 and the Gradle wrapper (bundled in `server/`)
+   - Frontend: Node.js 22+ and [pnpm](https://pnpm.io/)
+   - Optional: Docker + Docker Compose for the full-stack run
+3. Follow the [Quick start](README.md#quick-start) in the README to run the app
+   locally.
+
+## Development workflow
+
+- Create a feature branch from `main`, using a short prefix for clarity:
+  `feat/`, `fix/`, `refactor/`, `chore/` or `docs/` (e.g. `feat/role-audit-log`).
+- Keep pull requests small and focused on a single concern.
+- Write or update tests for the code you touch.
+- Open a pull request against `main` and describe what changed and why.
+
+## Code style
+
+The full conventions are documented in [AGENTS.md](AGENTS.md). The essentials:
+
+**Backend (`server/`)**
+
+- Follow the `Controller → Service → Manager → Repository` layering.
+- Use the custom validation framework for input validation.
+- Throw `BizException` with a `MessageName` constant and add the message key to
+  every bundle under `src/main/resources/i18n`.
+- Add Flyway migrations for schema changes under `src/main/resources/db/migrations/`.
+- Run `./gradlew build` before opening a PR.
+
+**Frontend (`web/`)**
+
+- Use ESLint and Prettier formatting; run `pnpm lint`.
+- Keep API clients in `src/api` and state in Redux Toolkit slices.
+- Never commit local `.env` files.
+
+## Commit messages
+
+Use [Conventional Commits](https://www.conventionalcommits.org/):
+
+```
+feat: support TOTP multi-factor authentication
+fix: resolve department name uniqueness check
+refactor: extract build cookie
+docs: update deployment guide
+chore(deps): upgrade spring boot to 3.5.4
+```
+
+## Reporting issues
+
+When opening an issue, include:
+
+- What you expected to happen and what actually happened
+- Steps to reproduce
+- Environment details (OS, Java/Node versions, deployment mode)
+
+Security issues: please report them privately to the maintainers rather than
+opening a public issue.
+
+## License
+
+By contributing, you agree that your contributions are licensed under the
+[MIT licence](LICENCE).