fix(lock): fail closed when Redis unavailable for financial operations

[BWM0223]

Summary

Fixes #33 - Distributed lock falls back to per-process in-memory map

Root Cause

LockService uses localLockMap when this.redis is falsy, giving no cross-instance protection for financial flows (recordPayment, escrow ops).

Fix

• acquireLock with failClosed=true (default) throws DistributedLockError when Redis unavailable
• Financial operations cannot proceed without distributed lock
• Non-financial ops can explicitly opt into local fallback
• Clear error logging distinguishes fail-closed from fallback