feat(helm): add support for multi-user BasicAuth mode

Adds multi-user-basic authentication mode to the helm chart alongside
existing basic (single-user) and oauth modes.

Multi-user BasicAuth mode enables:
- Pass-through authentication (credentials in request headers)
- Optional background operations using app passwords via Astrolabe
- Optional OAuth client credentials (uses DCR if not provided)
- Token encryption and persistent storage for background sync

Changes:
- values.yaml: Add auth.multiUserBasic configuration section
- deployment.yaml: Add ENABLE_MULTI_USER_BASIC_AUTH and related env vars
- secret.yaml: Add secret template for token encryption key and OAuth credentials
- pvc.yaml: Add PVC template for token database persistence
- _helpers.tpl: Add helper functions for secret/PVC names

Tested with:
  helm template --set auth.mode=multi-user-basic \
    --set auth.multiUserBasic.enableOfflineAccess=true \
    --set auth.multiUserBasic.tokenEncryptionKey=... \
    --set vectorSync.enabled=true

Related: Multi-user deployment support (ADR-020)

charts/nextcloud-mcp-server/templates/pvc.yaml

@@ -16,6 +16,24 @@ spec:
       storage: {{ .Values.auth.oauth.persistence.size }}
 {{- end }}
 ---
+{{- if and (eq .Values.auth.mode "multi-user-basic") .Values.auth.multiUserBasic.enableOfflineAccess .Values.auth.multiUserBasic.persistence.enabled (not .Values.auth.multiUserBasic.persistence.existingClaim) }}
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: {{ include "nextcloud-mcp-server.fullname" . }}-token-storage
+  labels:
+    {{- include "nextcloud-mcp-server.labels" . | nindent 4 }}
+spec:
+  accessModes:
+    - {{ .Values.auth.multiUserBasic.persistence.accessMode }}
+  {{- if .Values.auth.multiUserBasic.persistence.storageClass }}
+  storageClassName: {{ .Values.auth.multiUserBasic.persistence.storageClass }}
+  {{- end }}
+  resources:
+    requests:
+      storage: {{ .Values.auth.multiUserBasic.persistence.size }}
+{{- end }}
+---
 {{- if and (eq .Values.qdrant.mode "persistent") .Values.qdrant.localPersistence.enabled (not .Values.qdrant.localPersistence.existingClaim) }}
 apiVersion: v1
 kind: PersistentVolumeClaim