Default postgres/redis subcharts to off; document managed-DB path

[dhilgaertner]

Context

Today the chart enables Bitnami postgres as a subchart by default and has Bitnami redis as an optional dependency. On the OCI SaaS deployment we use OCI Database for PostgreSQL (managed) and we are not deploying Redis in cluster. For self-hosted we still want the in-cluster Bitnami postgres option available but not the default — most customers will want to point at their own managed DB.

Goal

Chart defaults to external DB references (no in-cluster DB) while keeping Bitnami postgres/redis available via values toggles.

Work items

• Set postgresql.enabled: false by default in values.yaml.
• Set redis.enabled: false by default.
• When both are off, DATABASE_URL must come from an existing Secret reference (add the ref pattern if not present).
• Update README.md with a "Choosing a database" section:
  • Managed: point database.externalDsnSecretRef at a Secret with database-url.
  • In-cluster: set postgresql.enabled=true.
• Ship a reference values-saas.yaml that uses an external DSN (ESO-backed Secret from OCI Vault).

Acceptance

• Default helm install . refuses to start unless an external DSN Secret is referenced (clear error).
• helm install . --set postgresql.enabled=true still works identically to today.

Dependencies

• Stacks with the "values surface" ticket (Add values surface for Stripe, OIDC, BYOK encryption key #14).

Key files

• values.yaml
• templates/secrets.yaml / templates/deployment.yaml
• README.md