feat: add Helm chart, docs and automated releases - #3

Summary

• Harden conditional range responses and add Go checks.
• Add a minimal Helm chart with configurable replicas, health probes, optional Ingress and an existing Secret for S3 credentials.
• Add a container build and documentation.
• After a verified push to main, create a semantic version tag and GitHub release, publish a matching Helm chart archive and multi-architecture GHCR image.

Validation

• Go test, vet and build passed locally.
• Helm lint, template with three replicas and Ingress, and versioned package passed locally.
• Docker build requires the PR CI runner because the local Docker daemon is unavailable.