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/Repository/Catalog.php

@@ -88,6 +88,11 @@ final class Catalog {
 	/** Write the next generation after the in-memory state was changed. */
 	public function write(): void {
 		$next = $this->gen + 1;
+		// Two writers starting from the same generation would otherwise overwrite each other
+		// silently and break the chain for whichever server anchored the lost write.
+		if ($this->backend->exists($this->path($next))) {
+			throw new RepositoryException("Catalog generation $next was written by another process at the same time; try again");
+		}
 		$plain = json_encode(['repository' => $this->repositoryId, 'gen' => $next, 'time' => gmdate('c'), 'prev' => $this->hash,
 			'snapshots' => array_keys($this->snapshots), 'trash' => array_keys($this->trash),
 			'userExports' => array_keys($this->userExports)], JSON_THROW_ON_ERROR);