Add Redis-backed multi-instance support

[darthfork]

Summary

• add Redis-backed delivery deduplication and shared workflow/job state hooks for multi-instance deployments
• add Redis configuration to the service and document Docker, binary, and Helm usage
• add an optional Redis Helm dependency and chart values for one-stop Kubernetes deployment

Notes

• bundled Redis is exposed through chart configuration with redis.enabled=true
• helm dependency update helm/promgithub currently depends on registry access in the packaging environment

Testing

• make lint
• go test ./...

Closes #45