fix(visu): enforce admin auth on visu mutation endpoints

[Micsi]

Motivation

• Fix a security regression where frontend-only admin checks (based on localStorage) could be bypassed because server-side Visu mutation endpoints used get_current_user (authenticated-only) instead of an admin check.
• Ensure that any API that creates, updates, deletes, moves/copies nodes or saves page configs requires real admin authorization on the backend.

Description

• Replaced Depends(get_current_user) with Depends(get_admin_user) for mutating endpoints in obs/api/v1/visu.py so that the following endpoints now require admin privileges: POST /nodes/import, POST /nodes, PATCH /nodes/{node_id}, DELETE /nodes/{node_id}, POST /nodes/{node_id}/copy, PUT /nodes/{node_id}/move, and PUT /pages/{node_id}.
• The change is localized to obs/api/v1/visu.py and does not alter read-only endpoints or page access resolution logic.
• This is a minimal, behavior-preserving fix that hardens server-side authorization without changing client code or API shapes.

Testing

• Compiled the modified module with python -m compileall obs/api/v1/visu.py and the compilation succeeded.
• Basic repository grep/inspection confirmed the targeted function signatures were updated to use Depends(get_admin_user).

---

Codex Task