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

main.rs

@@ -20,6 +20,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let sender = EmailSender::new(config.smtp)?;
     let (queue, worker) = match config.queue_capacity {
+        // `Some(0)` explicitly opts into an unbounded queue (mainly for testing).
         Some(0) => {
             info!("creating unbounded queue by explicit configuration");
             EmailQueue::unbounded()