Repair Docker Compose full-stack setup

[AlexKitipov]

Motivation

• Make docker compose up bring up a coherent local stack including backend, frontend, Postgres, Redis and an optional Nginx proxy.
• Ensure migrations run automatically before the API starts and use a sync PostgreSQL URL because the backend uses sync SQLAlchemy/psycopg2.
• Avoid exposing database/cache ports by default to keep Compose runs production-like and document how to publish them when needed.

Description

• Added a production-style frontend image with Dockerfile.frontend that builds the React app and serves the static bundle via Nginx and docker/frontend.conf.
• Reworked docker-compose.yml to include postgres, redis, a one-shot migrate job that runs /app/scripts/migrate.sh, backend, frontend, and an optional nginx service under the proxy profile, and made the backend depend on the migrate job via service_completed_successfully.
• Switched Compose and .env.example to a sync DB URL (postgresql://...) and set internal redis://redis:6379, removed default ports publishing for Postgres and Redis, and added published web ports (BACKEND_PORT, FRONTEND_PORT, NGINX_PORT).
• Updated Dockerfile to set PYTHONUNBUFFERED/PYTHONDONTWRITEBYTECODE, add EXPOSE 8000, and note that Compose runs migrations via the migrate job; added .dockerignore; and replaced the README Docker section with a new Docker Compose Full-Stack Setup that documents usage and override guidance.

Testing

• Ran npm run build in the frontend and the React production build completed successfully.
• Ran python -m compileall app to verify Python modules compile successfully.
• Loaded docker-compose.yml with a small Python script and asserted structure/behaviour (no ports on postgres/redis, backend depends on migrate, and DATABASE_URL starts with postgresql://), which passed.
• Attempted docker compose config but it could not run in this environment because Docker is not installed (/bin/bash: line 1: docker: command not found).

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Codex Task