```
refactor: update settings and Docker config for production
- Configure ALLOWED_HOSTS and CORS from environment variables for better security
- Switch default database from PostgreSQL to SQLite3 (PostgreSQL config commented)
- Simplify DEBUG environment variable handling
- Update Dockerfile to use Python 3.11 and gunicorn for production
- Add static file collection in Docker build process
- Add user appointment statistics endpoint (user_apointment_stats)
- Add .dockerignore to exclude unnecessary files from build
These changes improve production readiness by making critical settings
configurable via environment variables and using production-grade WSGI
server (gunicorn) instead of Django development server.
