A membership of listing_id in group_id, hydrated for the package editor and booking flow. A listing may belong to several groups. package_price (minor units) is the per-listing override when the group is a package, read from the group dimension of listing_prices (not a column on the join table): null means no override (use the listing's own price), 0 means explicitly free in the package, and a positive value overrides the price. quantity (≥1, stored on the join row) is how many of this listing one unit of the package includes. Both are ignored for non-package groups.