Add `/api/v2/userdata` endpoint

[benceruleanlu]

Second iteration of the /api/userdata endpoint. Now named /api/v2/userdata

Given an optional path query parameter, walks the specified sub‑directory of a user’s data directory and returns a JSON array of entries. Each entry is an object with:

• name: file or directory name
• type: "file" or "directory"
• path: relative path under the user root
• size & modified (for files only)

Ref: https://github.com/comfyanonymous/ComfyUI/pull/7759#pullrequestreview-2795134111