#509 [ENGINE/Memory] Unbounded tenant config cache cleanup

**File:** engine/config.py:593-610 **Problem:** The tenant config cache can grow unbounded if tenants are added faster than cleanup runs. Eviction only triggers at max size with a gap between cleanup and eviction. **Severity:** HIGH **Fix:** Add more aggressive cleanup when cache > 50% full: ```python def get_config(tenant_id: str | None = None) -> Config: with _config_lock: if len(_tenant_config_instances) > _TENANT_CONFIG_MAX_SIZE // 2: _cleanup_expired_tenant_configs() ``` **Impact:** Long-running workers may accumulate stale tenant configs.