Upsert the per-org config for one notification type.
event_type is a path param. severity_override is validated server-side against NotificationSeverity; threshold_config against the type's schema.
Successful Response
A catalog type merged with this org's config and resolved destinations.
Built by NotificationCatalogService (not from a single ORM row), so it surfaces both the static catalog metadata and the org's live state. The payload_schema / threshold_config_schema are JSON Schema (the variables/validation manifest the frontend hints from).