Filter coins by Bollinger Band rating.

    Args:
        exchange: Exchange name like KUCOIN, BINANCE, BYBIT, MEXC, etc.
        timeframe: One of 5m, 15m, 1h, 4h, 1D, 1W, 1M
        rating: BB rating (-3 to +3): -3=Strong Sell, -2=Sell, -1=Weak Sell, 1=Weak Buy, 2=Buy, 3=Strong Buy
        limit: Number of rows to return (max 50)

    Returns ``list[dict]`` on success. On ANY failure returns a structured
    error envelope ``{"error": {"code": ..., "retryable": ...}}``.