fix(db-backup): read the off-host leg of the dump on its own

A run that kept its dump on the volume and failed the upload is the newest
good dump and the newest failure at the same instant. Compared with ">", the
card showed no failure and readiness said fresh, night after night, while no
dump left the host.

The health read now carries the off-host leg separately: when the newest dump
reached the bucket, and the newest one that did not. One pure reading of it
(databaseDumpProblems) feeds both the backups card and the readiness model.
A configured bucket that missed the newest dump is attention (offhost_failed)
with the upload's error, and failing once the newest dump there is older than
36 hours (offhost_stale) or none ever arrived (offhost_never). The card shows
the refused upload and when the bucket last got a dump. A failure at the
instant of the newest dump for another reason, retention throwing, is now
shown too.

src/app/api/admin/backups/__tests__/route.test.ts

@@ -74,6 +74,7 @@ const databaseHealth = {
     offhost: true,
   },
   lastFailure: null,
+  offhost: { lastUploadAt: "2026-09-21T23:31:40.000Z", failure: null },
 };
 vi.mock("@/lib/database-backup/health", () => ({
   getDatabaseBackupHealth: async () => databaseHealth,