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>

src/app/metrics/route.ts

@@ -0,0 +1,16 @@
+// ABOUTME: Prometheus metrics endpoint at /metrics for standard scraping path.
+// ABOUTME: Re-exports the same metrics as /api/metrics for Prometheus autodiscovery.
+
+import { NextResponse } from "next/server";
+import { metricsRegistry } from "@/lib/metrics";
+
+export async function GET(): Promise<NextResponse> {
+  const metrics = await metricsRegistry.metrics();
+
+  return new NextResponse(metrics, {
+    status: 200,
+    headers: {
+      "Content-Type": metricsRegistry.contentType,
+    },
+  });
+}