Blended cost per litre

Petronas buys fuel from different suppliers in batches of different sizes and different prices, then sells it as one undifferentiated product at the pump. Finance wants the true blended cost per litre for each fuel grade — not a simple average of the batch prices, which ignores that a 50,000-litre batch and a 500-litre batch shouldn't count equally.

From fuel_batches, return each grade, and blended_cost_per_litre — the volume-weighted average cost per litre (total cost across all batches of that grade, divided by total litres of that grade), rounded to 4 decimal places. Order the output by grade.