Aggregate lead totals and breakdowns by stage and source

Returns flat totals (count, estimated_value_total) plus a breakdown by pipeline stage (with stage name joined from lead_stages) and a breakdown by source. totals.count counts every matching lead; totals.estimated_value_total and each by_source[].estimated_value_total are the OPEN pipeline value — leads in a Won or Lost stage are excluded, matching the app's Pipeline Value. by_stage[] reports each stage's own sum (a Won/Lost bucket carries its closed value) and flags closed stages with isWon / isLost. Null source becomes its own bucket. Money values are decimal strings. Date filter applies to createdAt.