Back up file uploads with the database

Some apps store uploaded files outside Postgres. Extend a project's backup to cover both halves, the DB dump and a file archive, through the same encrypt, local, R2, retention, and restore machinery.

Confirmed case: Issues, host path /home/james/issues/prod/data/uploads (a bind mount on issues-production-api-1). Apps with no files are unaffected and stay DB-only. This is layered on top, so the rest of the tool stays DB-only.

Done when: for a labelled app, bur backup also produces app-<ts>.files.tar.zst.age; bur restore puts both the DB and the files back; and --drill verifies both.