✨ feat(registration): add self-registration configuration option

- introduce ALLOW_SELF_REGISTRATION environment variable
- update .env-example and docker-compose.yml to include new setting
- modify registration logic to respect self-registration configuration

📝 docs(README): document self-registration configuration

- add description of ALLOW_SELF_REGISTRATION setting in README
- explain default value and its impact on user registration process

docker-compose.yml

@@ -13,6 +13,8 @@ services:
       - SESSION_COOKIE_SECURE=${SESSION_COOKIE_SECURE:-1}
       # Optional: encryption key for management passwords (Fernet key)
       - ENCRYPTION_KEY=${ENCRYPTION_KEY:-}
+      # Allow self-registration after first admin (1 = allow, 0 = only admin-created)
+      - ALLOW_SELF_REGISTRATION=${ALLOW_SELF_REGISTRATION:-0}
     volumes:
       - ./data:/app/data
     restart: unless-stopped