Prepare v0.5.0-beta production selfhosting foundation

[DeepZone]

Motivation

• Provide a clear, production-oriented self-hosting path for RouteForge (v0.5.0-beta) based on Docker Compose with a persistent PostgreSQL backend and operational guidance.
• Keep the application read-only and API-compatible while adding operationally useful healthchecks, documentation and a production frontend image.

Description

• Add docker-compose.prod.yml with postgres, backend and frontend services, a persistent routeforge_postgres_data volume, unless-stopped restart policies, and healthchecks for Postgres and the backend (and a lightweight frontend check).
• Replace the frontend Dockerfile with a multi-stage production build (Vite build stage + Nginx runtime) and expose the runtime on port 80 with build ARG/ENV for VITE_API_URL.
• Expand and comment .env.example for selfhosting (Postgres variables including POSTGRES_PASSWORD=change-me, DATABASE_URL, CORS_ORIGINS, VITE_API_URL, RIPEstat settings and LOG_LEVEL).
• Keep /health compatible while adding version metadata, bump the app/package versions to v0.5.0-beta / 0.5.0, and update UI text to reflect the new version.
• Add operations documentation under docs/operations/ for backup/restore, reverse proxy examples, logging and upgrades, and update README.md, ROADMAP.md and RELEASE_NOTES.md to document the selfhosting foundation and known limitations (notably beta-grade migrations).

Testing

• Backend unit tests were run with cd backend && pytest -q and succeeded (73 passed, 47 warnings).
• Frontend production build was validated with cd frontend && npm run build and completed successfully.
• docker compose -f docker-compose.prod.yml config could not be executed in the current environment because the Docker CLI is not available, so deployment config validation is skipped here.

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Codex Task