fix(proxy): strengthen upstream retry with exponential backoff
- Retry up to 3 times (was 1) for 429/500/503 responses
- Exponential backoff: 2s → 4s → 8s (was fixed 1s)
- Respect retry-after header with 20s cap (was 5s)
- Add ±20% jitter to avoid thundering herd
- Log each retry attempt when CC_VR_DEBUG=1