Win-back cancel survey

When someone cancels outside the app, it asks why on next open. Pick "too expensive" and it tells a short story from the user's own stats (words saved, days practised, hours of conversation), then unboxes a personalised discount.

• The call offer is one Remote Config key, with one showing event and one outcome event.
• Also built: the post-purchase questionnaire. Question sets come from Remote Config, with multi-select, an "Other" option that opens a free-text answer, and an admin preview mode that never saves responses.