Smart volume + technical analysis combination scanner.

    Args:
        exchange: Exchange name
        min_volume_ratio: Minimum volume multiplier (default 2.0)
        min_price_change: Minimum price change percentage (default 2.0)
        rsi_range: "oversold" (<30), "overbought" (>70), "neutral" (30-70), "any"
        limit: Number of results (max 30)

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