Add NixOS flake with dev shell, package, and systemd module
Includes nix develop shell (node, pg, vips), buildNpmPackage derivation, and a NixOS module for deploying as a systemd service with hardening, dedicated user, and configurable secrets via environmentFile. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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