Make MARC records available via central SFTP bucket

[tosteiner]

Is your feature request related to a problem? Please describe.
As indicated by librarians (Emma Booth and others), using the Thoth Export API as the source for MARC records can be problematic for library systems.

Describe the solution you'd like
As suggested by Emma Booth: Provide an SFTP bucket to access to daily / weekly / routinely-generated MARC dumps of all publishers' catalogues.
(This has come up again in the context of our metadata report, and I thought I'd document the need here ;) )