Return top gainers for an exchange and timeframe using Bollinger Band analysis.

    Args:
        exchange: Exchange name — crypto: KUCOIN, BINANCE, BYBIT, MEXC; stocks: EGX, BIST, NASDAQ, NYSE, BURSA, HKEX, SSE, SZSE, TWSE, TPEX
        timeframe: One of 5m, 15m, 1h, 4h, 1D, 1W, 1M
        limit: Number of rows to return (max 50)

    Returns:
        list[dict] on success. On ANY failure returns a structured error
        envelope ``{"error": {"code": ..., "retryable": ...}}`` — never a
        raw exception string.