feat(vault): deploy HashiCorp Vault 2.0.1 with raft HA (5 replicas)

StatefulSet with templated PVC (cephrbd-fast-delete, 10Gi), headless
service for raft cluster communication, HTTPS gateway (443→8200), and
kubernetes provider retry_join for automatic cluster formation.

apps/base/vault/namespace.yaml

@@ -0,0 +1,5 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: vault