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/Service/AlertService.php

@@ -54,6 +54,13 @@ final class AlertService {
 			['kind' => $run->getKind(), 'target' => $targetName, 'error' => mb_strimwidth((string)$run->getError(), 0, 300, '…')]);
 	}
 
+	/** A backup that finished but is missing unreadable files, or holds files that changed while read. */
+	public function runWarnings(Run $run, string $targetName, int $unreadable, int $changing, array $examples): void {
+		$this->notifyAdmins('run_warnings', 'run-' . $run->getId(), ['target' => $targetName,
+			'unreadable' => (string)$unreadable, 'changing' => (string)$changing,
+			'examples' => mb_strimwidth(implode(', ', array_merge($examples['unreadable'], $examples['changing'])), 0, 300, '…')]);
+	}
+
 	/**
 	 * Remember how much is stored on a location (for the settings page) and warn administrators
 	 * once a day when it is at 80% of its limit or more.