fix(helm): add backward compatibility for legacy persistence configs

- Add helper functions to detect and use legacy persistence configs
- Legacy auth.multiUserBasic.persistence.* and qdrant.localPersistence.*
  configs continue to work but show deprecation warnings in NOTES.txt
- New dataStorage.enabled takes precedence when explicitly set
- PVC size/accessMode/storageClass values from legacy configs are honored

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -300,7 +300,7 @@ spec:
             claimName: {{ include "nextcloud-mcp-server.oauthPvcName" . }}
         {{- end }}
         - name: data-storage
-        {{- if .Values.dataStorage.enabled }}
+        {{- if eq (include "nextcloud-mcp-server.dataStorageEnabled" .) "true" }}
           persistentVolumeClaim:
             claimName: {{ include "nextcloud-mcp-server.dataStoragePvcName" . }}
         {{- else }}