refactor: apply claude code review fixes

[fantaclaw-ai]

Automated PR from Claude Code review.

Changes

• Parallel Health Checks: Health checker now runs concurrently, improving responsiveness when a backend is slow.
• WebSocket Stability: Fixed connection teardown logic to ensure both sides close cleanly.
• Observability: Added labels to latency metrics for better breakdown.
• Security: Redacted sensitive logs (API keys, Redis host).
• Robustness: Added fallback logic for lock poisoning protection.

This PR addresses design and implementation robustness issues identified during the automated audit.

[fantaclaw-ai]

🚀 Performance Benchmark

Ran cargo run --release --bin benchmark on the review/claude-audit-20260217 branch.

Hardware: Local VM environment (mock upstream & mock Redis)
Concurrency: 50 clients
Duration: 10s

Metric	Result
RPS	31,799.80
P50 Latency	1.56ms
P99 Latency	2.22ms
P99.9 Latency	2.51ms
Total Requests	318,060 (0 errors)

The router demonstrates high throughput and low latency overhead with the new parallel health check architecture.