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

queue.rs

@@ -156,6 +156,8 @@ impl EmailQueue {
         if *self.shutdown.tx.borrow() {
             return Err(QueueError::Closed);
         }
+        // NOTE: ID is consumed even if the send below fails (channel full, closed).
+        // The u64 space is large enough that this is inconsequential in practice.
         let id = self.next_job_id()?;
         self.status_store.set_queued(id);
         let job = EmailJob {