fix: install curl in frontend Dockerfile for healthcheck compatibility

- Added curl installation in Debian slim runner stage
- curl is required for healthcheck in docker-compose.yml
- curl is pre-installed in node:20-slim base image was incorrect assumption
- Healthcheck uses: curl -f http://localhost:3000/
- Cleaned up apt cache to keep image size minimal

frontend/Dockerfile

@@ -22,6 +22,9 @@ WORKDIR /app
 ENV NODE_ENV=production
 ENV NEXT_TELEMETRY_DISABLED=1
 
+# curl installieren (fuer Health-Check)
+RUN apt-get update && apt-get install -y --no-install-recommends curl && rm -rf /var/lib/apt/lists/*
+
 RUN addgroup --system --gid 1001 nodejs
 RUN adduser --system --uid 1001 nextjs