feat: add Nix flake package definition

[TomVollerthun1337]

Summary

Adds packages.default to the flake so logsmith can be consumed as an installable Nix package directly from the repo.

Usage

In a NixOS machine config:

inputs.logsmith.url = "github:otto-de/logsmith";
# ...
environment.systemPackages = [ inputs.logsmith.packages.${system}.default ];

Run without installing:

nix run github:otto-de/logsmith

Implementation

• Python env: python313.withPackages with boto3, pyqt6, ruamel-yaml from nixpkgs. Using nixpkgs' PyQt6 means Qt is properly linked in the Nix store - no LD_LIBRARY_PATH workaround needed (that hack is only for PyPI wheels, which the devShell still uses).
• Entry point: makeWrapper creates bin/logsmith pointing at app/run.py. The project has a non-standard dual-root import layout (from app.X and bare import arguments); this is handled by PYTHONPATH=$out/share/logsmith combined with Python's automatic sys.path[0] insertion for the script directory.
• AWS CLI: awscli2 is appended to PATH for SSO login (aws sso login is called at runtime).
• Version: read from pyproject.toml via builtins.fromTOML - no need to update the flake on version bumps.
• devShell: the existing uv/PyPI-based devShell is unchanged.

Also adds .envrc (use flake) and excludes .direnv/ and result from git.

[TomVollerthun1337]

Superseded by #692 (by @PaulKobow7536), which takes a more complete approach via uv2nix. Planning to contribute macOS support and a few quality fixes directly to that PR instead.