Add Nomad deployment configuration and CI/CD pipeline

- Add docker.nix for Nix-based Docker image builds
- Update flake.nix with dockerImage package output
- Add output: standalone to next.config.ts for production builds
- Add /metrics endpoint for Prometheus scraping
- Add Gitea Actions workflow calling shared deploy-nomad.yaml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

next.config.ts

@@ -1,7 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  output: "standalone",
 };
 
 export default nextConfig;