fix: Yahoo Finance 429 rate limiting breaks APAC sentinel #10
Description
Impact
Tokyo sentinel fails to fetch Hang Seng/Nikkei data when Yahoo returns 429.
Signal Source
sentinel_apac.py — HTTP redirect handling + rate limit detection
Current Behavior
Sentinel crashes or returns stale data on 429 response.
Expected
Exponential backoff + fallback to cached last-known value.
Rollback Plan
Revert to previous sentinel version; data gap acceptable for <1h.