checkout_started only fired for the "full" plan (plus a mostly-unreachable
sticky-CTA branch), so non-full-plan users reached payment without it ever
firing — PostHog showed fewer checkout_started (1 person) than the downstream
payment_initiated (3), which is impossible in a clean funnel.
Move it to a step-driven effect that fires once when the payment step is
reached, via any path (plan card, sticky CTA, or a prefilled pending plan),
re-arming if the user returns to plan selection. Fixes the Monetization and
Ödeme funnels at the checkout step and keeps plan -> checkout meaningful
(brand-selection drop-off now shows up instead of a trivial 100% step).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>