Wechat Oauth Qr Poll

Long-poll WeChat for the scan/confirm of the QR minted by /qr.

Returns one of: {"ok": True, token, user} — confirmed (also stashed for /poll) {"ok": False, "pending": True, "scanned": bool} {"ok": False, "error": "expired" | "cancelled" | "..."} The SPA re-calls on pending (each call already blocks ~25s upstream, so this is not a busy loop).