feat: optional UDS listener + health endpoints for Kubernetes deployment

[tlongwell-block]

Summary

Adds two opt-in features for deploying sprout-relay on Kubernetes with an Istio service mesh:

1. Optional Unix Domain Socket listener — set SPROUT_UDS_PATH to bind a UDS alongside TCP. Useful when a service mesh sidecar (e.g. Istio/Envoy) delivers traffic via a Unix socket while Kubernetes health probes need TCP.

2. Health endpoints — /_liveness (always 200) and /_readiness (checks MySQL + Redis connectivity with a 2s timeout).

Changes

File	What
sprout-db/src/lib.rs	Add Db::ping() — SELECT 1 health check
sprout-relay/src/config.rs	Add uds_path: Option<String> from SPROUT_UDS_PATH env var
sprout-relay/src/main.rs	serve_dual() — TCP always + optional UDS via tokio::select!, #[cfg(unix)] gated
sprout-relay/src/router.rs	/_liveness, /_readiness routes; ConnectInfo read from extensions (works on both TCP and UDS)
sprout-relay/src/state.rs	Add redis_pool field for readiness probe

Design decisions

• Zero behavior change when SPROUT_UDS_PATH is unset — existing deployments are unaffected
• UDS code gated with #[cfg(unix)] — compiles cleanly on all platforms; non-unix logs a warning if the env var is set
• Readiness checks run concurrently via tokio::join! with a single 2s outer timeout (not sequential)
• Safe UDS cleanup — only removes existing socket files on startup; errors if the path exists but is not a socket
• No new dependencies — uses existing deadpool-redis and tokio features
• /health endpoint preserved for backward compatibility

Testing

• cargo fmt ✅
• cargo clippy --all-targets --all-features -D warnings ✅
• cargo test -p sprout-relay — 88/88 ✅
• cargo test -p sprout-db — 54/54 ✅