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

proto/Dockerfile

@@ -4,6 +4,10 @@ FROM node:20-alpine AS builder
 
 WORKDIR /app
 
+# NEXT_PUBLIC_* vars are inlined at build time — must be passed as build args
+ARG NEXT_PUBLIC_SITE_URL
+ENV NEXT_PUBLIC_SITE_URL=$NEXT_PUBLIC_SITE_URL
+
 # Copy package files
 COPY package.json package-lock.json* ./