get variant bundle contributions

How much of one variant's demand comes from each bundle that contains it, over a window: one entry per bundle with the bundle's product and variant titles and its contribution, plus total_count and total_demand.

variant_id is in the path; sales_channel_id, start_date and end_date are all required. It uses the same effective-demand figures as get_sales_velocity_table. At most 20 bundles come back, largest contribution first, and where there are more the last entry is an Others row summing the rest.

data_source says which store served the read, ch for the analytics mirror and pg for PostgreSQL, exactly as on get_sales_velocity_table. The figures are the same either way; the field is there because a fallback to PostgreSQL is otherwise invisible.

Scope: sales:read.