feat(01-01): Next.js app + Docker Compose stack with network segmentation

- Next.js 15 app with Tailwind CSS v4, standalone output for Docker
- Root layout with de locale, page with Tessera placeholder
- Multi-stage Dockerfile for web with monorepo root context
- Docker Compose with 4 services: traefik, web, api, db
- Three segregated networks: frontend-net, backend-net, data-net (internal)
- Traefik v2.11 reverse proxy routing / to web, /api to api
- API strip prefix middleware for clean routing
- PostgreSQL 16-alpine with health checks and named volume
- Fixed API Dockerfile for pnpm monorepo node_modules structure
- Fixed Next.js standalone path for monorepo (apps/web/server.js)
- Fixed Traefik Docker API version compatibility
- Network segmentation: web NOT on data-net, api NOT on frontend-net

apps/web/src/app/page.tsx

@@ -0,0 +1,10 @@
+export default function Home() {
+  return (
+    <main className="flex min-h-screen flex-col items-center justify-center">
+      <h1 className="text-4xl font-bold">Tessera</h1>
+      <p className="mt-4 text-lg text-gray-600">
+        Portal wird geladen...
+      </p>
+    </main>
+  );
+}