Phase 0: per-user backup/restore through user_migration (spike E)

RepositoryExportDestination / RepositoryImportSource implement the
public OCP\UserMigration interfaces on top of the repository, so
user_migration's migrators (calendar, contacts, trashbin, account,
files) export straight into the encrypted repository.

Spike E (NC34): export 0.8s, unchanged second export stores nothing;
after deleting the user, import restores account, files (bit-identical),
calendar event and contact. Findings: calendars/address books come back
as 'migrated-*' with new object URIs; importing over an existing user
duplicates contacts and adds a migrated birthday calendar; creating the
user on import needs >128 MB (password_policy word list).

lib/Repository/Repository.php

@@ -180,6 +180,16 @@ final class Repository {
 		}
 	}
 
+	/**
+	 * @param list<string> $blobIds
+	 * @return \Generator<string> verified blob contents, in order
+	 */
+	public function readBlobs(array $blobIds): \Generator {
+		foreach ($blobIds as $id) {
+			yield $this->loadBlob($id);
+		}
+	}
+
 	/** Small encrypted objects (manifests). */
 	public function putObject(string $path, string $data): void {
 		self::putString($this->backend, $path, $this->cipher->encryptString($data, 'object:' . $path));