Fix review findings: catalog writes, user restore consent, retention validation

Catalog: forgetting a user export or purging trash now writes the catalog
before deleting the objects, so a crash leaves at most an unreferenced file
instead of a catalog entry pointing at nothing. Per-user backup and the
safety export reopen the repository inside the catalog lock, so they build
on the latest catalog generation.

Commands: backup:user:backup and backup:user:restore resolve the location
before asking to install user_migration, and a successful install always
stops with a rerun instruction. backup:retention rejects a malformed
--user-last before changing any policy. backup:key:import fails clearly
when the passphrase file cannot be read.

lib/Service/RestoreFullService.php

@@ -110,6 +110,7 @@ final class RestoreFullService {
 			// directory: once applied, 'secret' may differ from what ng_backup's own Target
 			// options were encrypted with (see restoreFull()'s docblock), and every further
 			// repository access through $this->targets would fail to decrypt them.
+			/** @var array<string, mixed> $backupConfig */
 			$backupConfig = $this->fetchBackupConfig($target, $snapshotId);
 
 			$dataDir = rtrim($this->config->getSystemValueString('datadirectory'), '/');