Exposes two JSON endpoints under /api/v1/accounts/:id/captain/reports:
- GET /retention — aggregate KPIs (active/recurring/sleeping/at-risk/
churned, new vs returned in period, Pix generated/paid/conversion,
retention rates at 30d and 90d)
- GET /retention/cohort — monthly cohort matrix, 12 months lookback,
12 months of offset. Each cell is % of the cohort that interacted in
month M+N. SQL-aggregated with DATE_TRUNC + DISTINCT so it is a
single query even on large histories.
