Phase 0: repository on files_external storages without a mount (spike D)

ExternalStorageFactory builds storages with files_external's own
backends and auth mechanisms from an unsaved StorageConfig (as its
ConfigAdapter does), and StorageBackend adapts any IStorage to IBackend
(writeStream, seekable range reads, temp name + rename except on S3).
IBackend::put() now owns and always closes the stream, because
files_external's S3 writeStream closes it.

Spike D (NC34): S3 (SeaweedFS) and SFTP: 120 MiB backup ~6s, unchanged
run uploads nothing, restore bit-identical, no files_external mount
created, no temp files (max 12 KiB in /tmp). Works with files_external
disabled when its classes are loaded. Backup peak memory 85-89 MiB on
these targets (to investigate).

lib/Repository/Repository.php

@@ -111,7 +111,6 @@ final class Repository {
 		rewind($encrypted);
 		$this->backend->put('snapshots/' . $snapshotId, $encrypted);
 		fclose($tree);
-		fclose($encrypted);
 
 		return ['snapshot' => $snapshotId] + $stats;
 	}
@@ -272,7 +271,6 @@ final class Repository {
 		fwrite($s, $data);
 		rewind($s);
 		$backend->put($path, $s);
-		fclose($s);
 	}
 
 	private static function readFull($fh, int $length): string {