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

docker-compose.prod.yml

@@ -18,7 +18,11 @@ services:
       - tienda_network
     restart: unless-stopped
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-tienda_ilusion_user}"]
+      test:
+        [
+          "CMD-SHELL",
+          "pg_isready -U ${POSTGRES_USER:-tienda_ilusion_user} -d ${POSTGRES_DB:-tienda_ilusion_prod}",
+        ]
       interval: 10s
       timeout: 5s
       retries: 5