Pre-beta review: data safety in storage, catalog and backup runs

Storage and repository:
- Listing skipped every name containing ".part-", directories too, so the
  exports of a user like "john.part-time" were invisible to prune on S3,
  SFTP, FTP and SMB and their data was deleted. Only real temporary uploads
  ("<name>.part-<8 hex>") are skipped now.
- prune refuses to run while a recorded per-user export's manifest is
  missing, instead of freeing that export's data.
- forget() and untrash() copy first, switch the catalog, and delete the
  originals last, so a crash can no longer leave the catalog pointing at a
  missing snapshot or database dump.
- Writes are checked against the expected size, so a short upload is an
  error instead of a truncated pack; raw restores check rename() and touch().
- A catalog write refuses to overwrite a generation another process just
  wrote, and the catalog lock lasts 300 s, so opening a large repository
  inside it no longer lets the lease run out.

Backup runs and scheduling:
- A file that keeps changing is read again at most three times; then the
  last read is kept and reported, instead of restarting forever and
  blocking cleanup.
- Unreadable files and folders are recorded, logged, shown by occ
  backup:run, stored in the snapshot and notified to admins, instead of
  being skipped silently.
- After a failed scheduled backup, at most three attempts a day, an hour
  apart, instead of a new run and notification every five minutes.
- The job no longer spins on a run whose lease another process holds, and
  the weekly cleanup catches up after a Sunday spent backing up.
- Notifications for the per-user export retention events.

lib/Command/RunBackup.php

@@ -75,6 +75,13 @@ HELP);
 			});
 			if ($run->getStatus() === Run::DONE) {
 				$output->writeln(sprintf('  done in %.0fs', microtime(true) - $start));
+				$f = json_decode($run->getStats() ?? '{}', true)['files'] ?? [];
+				if (($f['unreadable'] ?? 0) > 0) {
+					$output->writeln(sprintf('<comment>  warning: %d unreadable files or folders are missing from this backup (see the server log)</comment>', $f['unreadable']));
+				}
+				if (($f['changing'] ?? 0) > 0) {
+					$output->writeln(sprintf('<comment>  warning: %d files kept changing and were saved as read (see the server log)</comment>', $f['changing']));
+				}
 			} else {
 				$output->writeln('<error>  ' . $run->getStatus() . ': ' . $run->getError() . '</error>');
 				$failed++;