⚠️ Major upgrade to Docs v6: the collaboration server is replaced by the new `yhub`
service, and existing documents have to be migrated to it, or they open EMPTY.

`SECRET_COLLAB_SS_VERSION` and the `collab_ss` secret are no longer used. The two
RSA keys the backend and yhub sign with are generated automatically on first
deploy into the `jwt_keys` volume (the `jwt-keys` service) and reused afterwards;
no manual secret insertion is needed.

After deploying (SOFT_MIGRATION is on by default), migrate all documents:

  abra app cmd <app-name> backend migrate_documents

Only then may you set SOFT_MIGRATION=false. See the README for details.