// POST /api/canvas
{
  "id": "abc123xyz",
  "url": "https://gui.new/abc123xyz",
  "edit_url": "https://gui.new/abc123xyz?token=sk_...",
  "sse": "https://gui.new/api/canvas/abc123xyz/events",
  "size": 2847,
  "expires_at": "2026-03-07T12:00:00Z",
  "created_at": "2026-03-06T12:00:00Z"
}