feat(k8s): add Kubernetes Helm chart for emailks service

- Create Helm chart structure with Chart.yaml and values.yaml
- Add deployment template with container configuration and environment variables
- Implement service template for gRPC port exposure
- Add service account template with security configuration
- Include horizontal pod autoscaler template for scaling capabilities
- Add helper templates for naming and label management
- Configure SMTP settings as configurable parameters in values.yaml
- Set up resource limits and requests for container performance
- Implement liveness and readiness probes for health checks
- Add support for existing secrets and custom configurations

config.rs

@@ -1,9 +1,8 @@
 use std::{env, fmt, net::SocketAddr, time::Duration};
 
 pub use crate::error::ConfigError;
+use crate::ENV_PREFIX;
 use tracing;
-
-const ENV_PREFIX: &str = "APP_SMTP_";
 const DEFAULT_LISTEN_ADDR: &str = "127.0.0.1:50051";
 
 #[derive(Debug, Clone, PartialEq, Eq)]