Keep the last good export until there is a new one

[blaipr]

The bug

XmlExport::export() deleted the previous export before it started writing the new one:

self::deleteExportFiles($exportPath->getPath());   // ← the old one is gone here
…
$this->buildAndSaveXml((string)$file, $password);  // ← and this can throw

So a run that failed part-way — an account it could not read, a failure writing the file — took the last good export with it and left the installation with none at all. The exception propagated correctly; there was simply nothing left behind it.

The existing tests already drove four of those failure paths. None of them had a previous export on disk to notice was missing.

The fix

Delete afterwards, skipping the file just written. A successful run still replaces the previous export — the directory does not accumulate every export ever taken — and a failed one leaves it where it was.

Testing

A test that seeds a previous export, makes the run fail, and asserts the previous one is still there. It fails against the old ordering.

It uses a real directory rather than the harness's virtual one, on purpose: the cleanup uses glob(), which does not see a stream wrapper, so against vfs the test would have passed while proving nothing.

The same ordering exists in the backup

BackupFile::doBackup() calls deleteOldBackups() before writing the new archives, with the same consequence for the database dump and the application archive. It is a larger change — the new archives are named from a hash the delete would also have to skip — and belongs in its own PR rather than being bolted onto this one.