Get Retention

An OPEN envelope around one report's answer.

The app wraps every one of these reports in {success, data, meta}, where meta carries the cache state, the window and - on the agency reports - failed_org_ids, the orgs that contributed NOTHING to the totals above. That field is the difference between "this client sent no email" and "we could not query this client", so it is forwarded rather than flattened away.

Open on purpose: data is dict | list in the app and its contents grow with the product. A closed model would turn every new metric into a silently dropped one.