4-step listener onboarding funnel for the trailing days.

Computes day-by-day signup → email-confirm → first-track-played → 2nd-day-return counts so the operator can baseline the funnel before the Stream 2a/2b UX redesigns ship + measure the delta after. See app/onboarding_funnel.py for the column semantics of each step.

Returns the per-day shapes plus a trailing-window summary so a single curl is enough to read the snapshot without post-processing on the operator side. Defaults to 14 days (matches the 2-week stabilization window); capped at 90 to keep the SQL bounded.