feat(redis): add Redis Sentinel support for HA cache discovery
- Add build_sentinel() function to discover Redis master via Sentinel
- Parse redis-sentinel://host:port,host2:port2/master_name URLs
- Query sentinels with SENTINEL GET-MASTER-ADDR-BY-NAME
- Add comprehensive logging for debugging discovery issues
- Support password auth and optional DB selection
- Route sentinel URLs from endpoint config path in cache.rs
- Warn when username/password/db are provided with sentinel URLs
- Fix redis feature gating with dep:redis