Cursor-paginated user-scoped document list with status-bucket filter.

Returns active documents for the supplied user_id, ordered (created_at DESC, id DESC). The opaque cursor is the next_cursor from the previous page (base64-url JSON tuple); malformed cursors return 400. The status query param buckets rows for the recovery surfaces: 'failed' (any layer failed), 'unsupported' (extraction marked unsupported), 'pending' (extraction or semantic_index in pending/running), or 'all' (default — every active row).