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/Notification/Notifier.php

@@ -42,6 +42,10 @@ final class Notifier implements INotifier {
 					: $l->t('Backup to %s failed', [$p['target']]));
 				$notification->setParsedMessage((string)($p['error'] ?? ''));
 				break;
+			case 'run_warnings':
+				$notification->setParsedSubject($l->t('Backup to %1$s finished with warnings: %2$s unreadable, %3$s changed while being read', [$p['target'], $p['unreadable'], $p['changing']]));
+				$notification->setParsedMessage($l->t('Unreadable files and folders are missing from this backup (check their permissions). Files that kept changing were saved as they were while being read. Examples: %s', [$p['examples'] ?? '']));
+				break;
 			case 'quota_warning':
 				$notification->setParsedSubject($l->t('Backup location %1$s is at %2$s%% of its limit (%3$s of %4$s)', [$p['target'], $p['percent'], $p['used'], $p['limit']]));
 				$notification->setParsedMessage($l->t('Backups stop when the limit is reached. Raise the limit, or let the retention policy remove older backups.'));
@@ -62,6 +66,8 @@ final class Notifier implements INotifier {
 					'catalog_trusted' => $l->t('%1$s accepted an older state of backup location %2$s (catalog generation %3$s)', [$who, $p['target'] ?? '', $p['generation'] ?? '']),
 					'slot_added' => $l->t('%1$s added backup passphrase slot %2$s (%3$s); download and confirm the new recovery kit', [$who, $p['slot'] ?? '', $p['label'] ?? '']),
 					'slot_replaced' => $l->t('%1$s set a new passphrase for backup key slot %2$s (%3$s); download and confirm the new recovery kit', [$who, $p['slot'] ?? '', $p['label'] ?? '']),
+					'user_export_retention_changed' => $l->t('%1$s changed how many exports per user are kept to %2$s', [$who, $p['keep'] ?? '']),
+					'user_exports_forgotten' => $l->t('%1$s removed %2$s per-user exports from %3$s', [$who, $p['count'] ?? '', $p['target'] ?? '']),
 					'slot_removed' => $l->t('%1$s removed backup passphrase slot %2$s; download and confirm the new recovery kit', [$who, $p['slot'] ?? '']),
 					default => $l->t('NG Backup security event'),
 				});