Optionaldocuments_OptionalfailedThe Anfragen whose last attempt failed and that have not been handled since, so
ka sync --retry-failed can take only them (issue #27). A ref leaves the list
when a later run stores it or finds it unchanged.
Per-URL ETag / Last-Modified, so unchanged documents are never re-fetched.
Optionallast_Last error message, kept so ka sources list can show a degraded source.
Optionallast_Optionallast_
Per-source bookkeeping: what was last seen, and the conditional-request state.