Wire Medusa backups in K8ssandraCluster template

[fbeneventi]

Context

values-production.yaml and the hardening documentation have
historically claimed Medusa backups are configured. The
K8ssandraCluster template
(deploy/helm/examon/templates/k8ssandra-cluster.yaml) wires only
Cassandra, Reaper, and telemetry. There is no spec.medusa block.

Impact

Operators following the docs end up with no automated backup. Manual
patching of the CR is required after install.

Desired behaviour

Add a spec.medusa block to the K8ssandraCluster template, gated by
cassandra.medusa.enabled. Support common storage backends (S3, GCS,
Azure Blob, Ceph/S3) via cassandra.medusa.storageProperties.
values-production.yaml should set sane defaults; values-staging.yaml
keeps Medusa disabled.

Current workaround (documented)

docs/administrators/deploy/harden-for-production.md § Backups
documents a post-deploy kubectl patch of the K8ssandraCluster CR
with a spec.medusa block. The misleading # Full HA, production resources, TLS, Medusa backups. header comment in
values-production.yaml has been stripped to # Full HA, production resources, TLS. pending the real wiring.

Files to touch

• deploy/helm/examon/templates/k8ssandra-cluster.yaml
• deploy/helm/examon/values.yaml (cassandra.medusa.* defaults)
• deploy/helm/examon/values-production.yaml
• deploy/helm/examon/values-staging.yaml

Doc cleanup when closed

• Rewrite docs/administrators/deploy/harden-for-production.md § Backups
  to point at the chart value instead of the manual patch.
• Update docs/administrators/deploy/staging.md Backups row in the
  staging-vs-production comparison block (currently reads
  "Medusa (manual patch in v0.5.0)").
• Remove the Medusa paragraph from § Known gaps in v0.5.0.
• Restore "Medusa backups" to the header comment of
  deploy/helm/examon/values-production.yaml (line 2:
  # Full HA, production resources, TLS, Medusa backups.).