#462 Memory Leak: Unbounded _tenant_config_instances in config.py

engine/config.py:584,598-605 has unbounded _tenant_config_instances dict. Each unique tenant_id creates a TenantConfig instance that is cached forever. In multi-tenant deployments with many tenants, this causes memory growth. FIX: Add TTL-based expiration or max_size limit with LRU eviction.