Returns custom data groups with their fields, paginated by group (default 1000, max 2000). data is an OBJECT keyed by group_key. All filters are optional; site and lang default to the main site and the default language. Business errors are returned with HTTP 200 and success: false; each message is prefixed with its code (CDxxxx - ...). On error the body is an ARRAY: [{"success": false, "errors": [...]}] (CD0026 invalid 0/1 filter, CD0051 unknown group_key, CD0079 unexpected field).