Returns custom data fields, sorted by key, paginated (default 1000, max 10000). 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": [...]}] (CD0030 language not active on the site, CD0079 unexpected field).