Phase 0: fixes from review (torn files, pending blobs, exact sequences, UTF-8, storage checks)

- detect files changing during reading or between resumable steps and
  re-read them (size/mtime/ctime/inode), spike F overwrites mid-run
- drop pending blob ids when a pack or index upload fails
- dump and restore PostgreSQL sequence state exactly, bound parameters,
  validated catalog names; spike C checks all sequences exactly
- keep invalid UTF-8 byte-exact (tagged base64)
- refuse large tables without a primary key
- StorageBackend: check write/flush/close results, validate base path
- user_migration import/export: root paths, intermediate folders,
  partial writes, read errors
- spikes: never touch an existing account, own fixtures, 512M note

lib/UserMigration/RepositoryExportDestination.php

@@ -11,6 +11,7 @@ use OCA\NgBackup\Repository\Repository;
 use OCP\Files\File;
 use OCP\Files\Folder;
 use OCP\UserMigration\IExportDestination;
+use OCP\UserMigration\UserMigrationException;
 
 /**
  * Export destination for user_migration that writes straight into the encrypted repository:
@@ -42,7 +43,7 @@ final class RepositoryExportDestination implements IExportDestination {
 		while (!feof($stream)) {
 			$chunk = fread($stream, 1048576);
 			if ($chunk === false) {
-				break;
+				throw new UserMigrationException('Read error while exporting ' . $path);
 			}
 			$writer->write($chunk);
 		}