[Feature]: Add user cloud summary endpoint for Vault Web dashboard

Problem

Vault Web should be able to show Cloud Page information on the user dashboard, but the frontend should not inspect filesystem details directly or depend on unsafe cross-service assumptions.

Example: a user opens the Vault Web dashboard and wants to see used storage, quota, recent cloud activity, and a quick link into their cloud area.

Proposed solution

Add a secure, dashboard-ready Cloud Page endpoint for the authenticated user, for example GET /api/dashboard/summary.

Suggested response fields:

• usedStorageBytes
• quotaBytes
• fileCount
• folderCount
• recentItems with name, path, type, size, and timestamp

A streaming endpoint can be considered later, but a small summary endpoint is enough for the first version.

Acceptance criteria

• Endpoint only returns data for the authenticated user.
• Paths are normalized and do not expose host/root filesystem paths.
• Response is small enough for dashboard usage.
• Tests cover authenticated access and user isolation.
• API docs or README mention the endpoint.