POST /boards/{slug}/export and POST /boards/import, so the desktop and
dashboard can drive board transfer. Both exchange filesystem paths
rather than bytes, the same contract profile export/import uses and for
the same reason: the client runs its native save/open dialog on the
machine that hosts the backend, so a path is all either side needs, and
a board carrying a few hundred megabytes of attachments never has to
cross the renderer heap.
Also covers the pre-existing rename (PATCH) and delete endpoints, which
had no tests — including that delete archives to a restorable directory
and refuses to touch `default`.
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