#510 [ENGINE/Memory] Vault secret cache entries never proactively removed

**File:** engine/config.py:82-83 **Problem:** Cache entries are checked for expiration on read but expired entries are never proactively removed. Over long worker lifetimes, cache accumulates thousands of expired entries never accessed again. **Severity:** HIGH **Fix:** Add periodic cleanup method: ```python def _cleanup_expired_vault_cache(self) -> None: current_time = time.time() with self._vault_cache_lock: expired_keys = [ key for key, (_, cached_at) in self._vault_cache.items() if current_time - cached_at > VAULT_CACHE_TTL_SECONDS ] for key in expired_keys: del self._vault_cache[key] ``` **Impact:** Memory growth in long-running workers.