fix: update docker-compose to docker compose and add ADMIN_PASSWORD to .env.example (#189)

- Replace deprecated docker-compose (v1) with docker compose (v2) in README
- Add missing ADMIN_PASSWORD to .env.example so new users don't get a
  blank-string warning and can actually log in after first setup

.env.example

@@ -22,6 +22,7 @@ REDIS_PASSWORD=your_secure_redis_password_here
 # Admin Account (initial setup)
 ADMIN_USERNAME=admin
 ADMIN_EMAIL=[email protected]
+ADMIN_PASSWORD=your_secure_admin_password_here
 
 # Email Configuration
 # For Gmail: use app-specific password