Allow using an external Redis/Valkey instance instead of bundled deployment

Scope:

• Add valkey.deploy flag (default true); when false, skip Deployment, Service, PVC

• Make valkey.host configurable (currently hardcoded in helper to <release>-valkey)

• Add optional valkey.existingSecret for auth (Redis AUTH password)

• Add TLS support for external Redis connections (valkey.tls.enabled, CA cert)

• Update Celery broker URL construction to handle auth + TLS

• Document required Redis configuration (memory policy, persistence expectations)

Estimate: High (no external support exists today, needs new values schema + template logic)