Generic Helm chart for self-hosting with bring-your-own secrets

The Helm chart has been genericized so any Hive deployment can use it. Defaults no longer bake in Tuist-specific host names, image pull secrets, External Secrets Operator wiring, or the Postgres S3 backup; those production settings now live in a values-production.yaml overlay applied via helm upgrade --values. A new secrets.existingSecret value lets operators point at their own Kubernetes Secret for SECRET_KEY_BASE and any OAuth credentials, and the README includes a copy-pasteable bring-your-own-secret recipe for self-hosters.