chore: add Nix flake-based development environment

Co-authored-by: Kumpelinus <kumpelinus@jat.de>

- Add Nix flake and lockfile for reproducible development environments
- Document Nix-based setup in `docs/nix-development.md`
- Add `.envrc` for direnv integration and update `.gitignore` for Nix/direnv artifacts
- Reference Nix setup in CONTRIBUTING.md

docs/nix-development.md

@@ -0,0 +1,71 @@
+# Nix Development Environment
+
+LLDAP provides a Nix flake that sets up a complete development environment with all necessary tools and dependencies.
+
+## Requirements
+
+- [Nix](https://nixos.org/download.html) with flakes enabled
+- (Optional) [direnv](https://direnv.net/) for automatic environment activation
+
+## Usage
+
+```bash
+# Clone the repository
+git clone https://github.com/lldap/lldap.git
+cd lldap
+
+# Enter the development environment
+nix develop
+
+# Build the workspace
+cargo build --workspace
+
+# Run tests
+cargo test --workspace
+
+# Check formatting and linting
+cargo fmt --check --all
+cargo clippy --tests --workspace -- -D warnings
+
+# Build frontend
+./app/build.sh
+
+# Export GraphQL schema (if needed)
+./export_schema.sh
+
+# Start development server
+cargo run -- run --config-file lldap_config.docker_template.toml
+```
+
+## Building with Nix
+
+You can also build LLDAP directly using Nix:
+
+```bash
+# Build the default package (server)
+nix build
+
+# Build and run
+nix run
+```
+
+## Development Shells
+
+The flake provides two development shells:
+
+- `default` - Full development environment
+- `ci` - Minimal environment similar to CI
+
+```bash
+# Use the CI-like environment
+nix develop .#ci
+```
+
+## Automatic Environment Activation (Optional)
+
+For automatic environment activation when entering the project directory:
+
+1. Install direnv: `nix profile install nixpkgs#direnv`
+2. Set up direnv shell hook in your shell configuration 
+3. Navigate to the project directory and allow direnv: `direnv allow`
+4. The environment will automatically activate when entering the directory