#473 Race Condition: Vault cache read-modify-write without lock

engine/config.py:227-285 Vault cache operations not protected by lock: 1. Cache read at line 227-245 (outside lock) 2. Cache write at line 282-284 (outside lock) Race: Thread A reads stale cached secret while Thread B updates cache. Impact: Security issue - stale credentials returned. FIX: Protect all _vault_cache operations with threading.Lock.