feat: Add staging environment for local production testing

- Add docker-compose.staging.yml with PostgreSQL and Django
- Add .env.staging.example with staging-specific environment variables
- Configure staging settings (DEBUG=False, no SSL redirect for localhost)
- Update settings/__init__.py to support staging environment detection
- Update AGENTS.md with staging environment documentation
- Update .gitignore to exclude .env.staging
- Optimize docker-compose.prod.yml configuration

Staging environment simulates production configuration locally:
- PostgreSQL database (port 5433 to avoid conflicts)
- Gunicorn with 4 workers
- DEBUG=False but HTTP allowed for easier testing
- Separate volumes for static files, media, and logs

Usage:
  cp .env.staging.example .env.staging
  docker compose -f docker-compose.staging.yml up -d

.gitignore

@@ -363,3 +363,4 @@ pgdata/
 # IDE-specific
 .vscode/
 .idea/
+.env.staging