Add flake.nix providing:

- packages.default: Quickwit 0.8.0 binary for x86_64-linux and aarch64-linux
  (fetched from GitHub releases, patched with autoPatchelfHook)
- devShells.default: development shell with rustup, protobuf, cmake,
  pkg-config, openssl, nodejs, and yarn
- nixosModules.quickwit: NixOS module with systemd service, including:
  - Dedicated quickwit user/group
  - Configurable listen address, port, and data directory
  - YAML settings merged into quickwit.yaml
  - systemd hardening (NoNewPrivileges, ProtectSystem, etc.)
  - Optional firewall rules for HTTP (7280) and gRPC/gossip (7281)

Usage:
  nix build          # build quickwit binary
  nix develop        # enter dev shell
  nix flake show     # list all outputs

NixOS configuration example:
  services.quickwit = {
    enable = true;
    listenAddress = "0.0.0.0";
    openFirewall = true;
    settings.peer_seeds = [ "node-2.local" ];
  };