Add beginner VPS deployment guide

[Spbd1]

Motivation

• Provide a clear, beginner-friendly walkthrough for deploying the app to an Ubuntu 22.04/24.04 VPS using Docker Compose and a reverse proxy so newcomers can run production pilots safely.

Description

• Add docs/VPS_DEPLOYMENT.md containing step-by-step instructions for server update, installing required tools, Docker official apt repository setup (keyring and repository), installing docker-ce, docker-ce-cli, containerd.io, docker-buildx-plugin, and docker-compose-plugin, and verifying docker --version and docker compose version.
• Document cloning the specific branch with git clone --branch codex/improve-admin-dashboard-analytics ..., creating and editing .env from cp .env.example .env, required production .env values (APP_BASE_URL, ENABLE_SERVER_SUBMISSION, NEXT_PUBLIC_ENABLE_SERVER_SUBMISSION, ADMIN_EXPORT_TOKEN, POSTGRES_PASSWORD, plus optional GOOGLE_SHEETS_WEBHOOK_URL/GOOGLE_SHEETS_WEBHOOK_SECRET), and generating strong secrets with openssl rand -hex 32.
• Include deployment and operational commands: docker compose up --build -d, docker compose exec app npm run db:migrate, health checks (docker compose ps, docker compose logs -f app, curl http://127.0.0.1:3000/api/health), a beginner-friendly Caddy reverse-proxy example, UFW firewall commands, update workflow, and backup/restore notes for the postgres_data Docker volume including a pg_dump example docker compose exec -T postgres pg_dump -U hcg -d hidden_cost_game > backups/hidden_cost_game_$(date +%F_%H%M%S).sql.
• Add a short link to docs/VPS_DEPLOYMENT.md from the Docker section in README.md.

Testing

• Ran git diff --check which reported no issues.
• Verified the new documentation and README are readable using a node -e file-read check which succeeded.
• Attempted to run markdownlint but it was not installed in the environment, so markdown linting was skipped.

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Codex Task