feat: helm chart for kubernetes deployment

[kacy]

summary

• adds a minimal helm chart under helm/ember/ for deploying ember to kubernetes
• chart creates a deployment, service, and serviceaccount
• all ember configuration flows through EMBER_* environment variables, matching the existing dockerfile support
• adds helm-lint and helm-template targets to the makefile
• adds a kubernetes section to the README with install examples

what was tested

• helm lint helm/ember — passes clean (only an info about missing icon)
• helm template ember helm/ember — renders valid manifests with default values
• helm template with overrides (--set ember.maxMemory=512M, --set ember.appendonly=true, etc.) — all conditionals render correctly
• verified serviceaccount is omitted when serviceAccount.create=false

design considerations

• kept the chart minimal: no ingress, PVC, or HPA. users can extend as needed
• uses emptyDir for /data — persistence in k8s would need a PVC, which is out of scope for v1
• metrics port is conditionally exposed in both the deployment and service
• TCP probes on the cache port for liveness/readiness (no HTTP endpoint needed)