rekey method
Re-encrypts the store at rest under newKey, keeping all data.
Called when the group key rotates (device revocation). In-memory engines
hold no at-rest key, so the default is a no-op; a SQLCipher-backed engine
runs PRAGMA rekey.
Implementation
@override
Future<void> rekey(List<int> newKey) => _inner.rekey(newKey);