[Feature]: Implement secure file and folder sharing between users

Problem

Cloud Page users cannot securely share selected files or folders with other registered users yet.

Example: Alice wants to share a PDF or project folder with Bob without giving Bob access to the rest of her cloud storage.

Proposed solution

Implement backend support for sharing files and folders between registered Cloud Page users.

The first version should focus on the secure API and data model. Vault Web can add a Shared with me UI after the backend contract exists.

Scope:

• add a normalized share model/table for files and folders
• support permissions such as view, download, and optionally edit
• add endpoints to create, list, and revoke shares
• add an endpoint for items shared with the current user
• enforce ownership and permission checks on every shared operation

Acceptance criteria

• Users can only share items they own.
• Recipients can only access explicitly shared items.
• Folder sharing handles nested files safely without path traversal leaks.
• Revoked shares stop access immediately.
• Tests cover owner access, recipient access, denied access, and revoked access.
• API docs or README describe the sharing endpoints.