feat(registry): add service discovery and health check capabilities

- Integrate tonic-health for gRPC service health monitoring
- Add etcd-based service registration with automatic keep-alive
- Implement dynamic configuration loading from etcd with fallback
- Remove external dependencies from docker-compose for simplified deployment
- Refactor service registration logic with improved lease management
- Add health service to gRPC server with serving status reporting

etcd/discovery.rs

@@ -22,6 +22,7 @@ impl EtcdRegistry {
             self.load_initial("mail").await?;
             self.spawn_watch("mail");
         }
+
         Ok(())
     }