Alpha-cohort retention funnel.

Six steps over the last N days:

1. subscribed — listeners with a row in listener_subscriptions
2. queued — at least one Article matched their verticals
3. played — at least one ListenerHistory row
4. completed — at least one ListenerHistory.completed=true
5. returned — listened on >=2 distinct UTC days
6. upgraded — flipped free → plus / pro inside the window (monetization audit Action #5, 2026-05-31). Reads listener_subscriptions.tier_upgraded_at (added in migration 0143; backfilled from plus_since so historical paid listeners aren't blanked-out).

The point is signal: how many invitees actually came back, and how many of those CONVERTED? The drop-off between stages tells you which gate to invest in next (intake quality, queue UX, audio quality, repeat-listening hooks, monetization wedge).