feat(deploy): add staging environment with dedicated docker compose

- Add docker-compose.staging.yml (app on 127.0.0.1:8081, separate MongoDB
  on 127.0.0.1:27019, dedicated volume/network)
- deploy.sh: select compose file and port per environment
  (staging 8081, production 8080)
- Dockerfile: accept NEXT_PUBLIC_SITE_URL as build arg (inlined at build
  time for NEXT_PUBLIC_ vars); pass it in both compose files
- Add .env.staging.example and .env.production.example

docker-compose.staging.yml

@@ -0,0 +1,54 @@
+services:
+  # Next.js Application (Staging Mode)
+  app:
+    build:
+      context: ./proto
+      dockerfile: Dockerfile
+      target: runner
+      args:
+        - NEXT_PUBLIC_SITE_URL=${NEXT_PUBLIC_SITE_URL:-https://stage.mozdit.hu}
+    container_name: mozdit-app-staging
+    ports:
+      - "127.0.0.1:8081:3000" # Belső port — csak nginx-en keresztül elérhető
+    environment:
+      - NODE_ENV=production
+      - MONGODB_URI=${MONGODB_URI:-mongodb://mongodb:27017/mozdit}
+      - MONGODB_DB=${MONGODB_DB:-mozdit}
+      - NEXT_PUBLIC_SITE_URL=${NEXT_PUBLIC_SITE_URL:-https://stage.mozdit.hu}
+      - NEXT_PUBLIC_COMPANY_NAME=${NEXT_PUBLIC_COMPANY_NAME:-mozdIT Bt.}
+      - NEXT_PUBLIC_CONTACT_EMAIL=${NEXT_PUBLIC_CONTACT_EMAIL:-info@mozdit.hu}
+      - LOKI_HOST=${LOKI_HOST:-http://loki:3100}
+    depends_on:
+      - mongodb
+    networks:
+      - mozdit-network-staging
+    restart: always
+    logging:
+      driver: "json-file"
+      options:
+        max-size: "10m"
+        max-file: "3"
+
+  # MongoDB Database (külön a prod-tól)
+  mongodb:
+    image: mongo:7.0
+    container_name: mozdit-mongodb-staging
+    ports:
+      - "127.0.0.1:27019:27017" # Belső port, nem ütközik a prod 27018-cal
+    environment:
+      - MONGO_INITDB_ROOT_USERNAME=${MONGO_ROOT_USER:-admin}
+      - MONGO_INITDB_ROOT_PASSWORD=${MONGO_ROOT_PASSWORD:-password123}
+      - MONGO_INITDB_DATABASE=mozdit
+    volumes:
+      - mongodb_data_staging:/data/db
+    networks:
+      - mozdit-network-staging
+    restart: always
+
+volumes:
+  mongodb_data_staging:
+    driver: local
+
+networks:
+  mozdit-network-staging:
+    driver: bridge