Add Helm chart for Kubernetes deployment

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Summary

• Add charts/spectogram: a Helm chart deploying the serve container as a Deployment + Service, with optional Ingress, HPA, and an opt-in generate Job (runs the Go CLI against a ConfigMap-provided audio file, writing to the same PVC the Deployment mounts)
• Designed for zero-template-change portability: default values (ClusterIP, chart-managed PVC on the cluster's default StorageClass, no Ingress) work unmodified on kind; overriding image.repository/image.tag, service.type, ingress.*, and persistence.storageClassName covers cloud/on-prem
• Add a helm test hook Pod that fetches /web/index.html through the Service and asserts the Plotly viewer is served
• Add .github/workflows/helm.yml: lints and templates the chart on every relevant PR, and additionally builds the image, spins up a real kind cluster, runs helm install --wait, and runs helm test
• Document chart usage (charts/spectogram/README.md) and link it from the top-level README.md

Test plan

• helm lint and helm template (default values, and all optional features enabled) pass locally
• Manually validated end-to-end on a local kind cluster: built the image, installed the chart, confirmed the PVC bound, ran the generate Job against a synthesized WAV via a ConfigMap, confirmed the output landed on the shared PVC and was served through the Service, and helm test passed
• go build, go vet, gofmt, go test -race -cover still pass
• actionlint passes on the new workflow
• CI's kind-install job (same flow, automated) passes