Parse the top-level `attachments` array in WeKan board JSON exports,
group them by card ID, base64-decode the payload, and attach the
resulting files to the generated tasks so they land in Vikunja as
task attachments. Orphaned attachments (cardId with no matching card)
are silently skipped; decode errors are logged and skipped.
Add comprehensive tests for the WeKan conversion function including
edge cases (empty board, orphan cards, color mapping, multiple
checklists, unsupported fields) and a realistic JSON fixture file.
kolaente