exists method
Returns true if the keystore currently contains key,
else false. The backend-agnostic existence check — the same
call site works against every backend.
Should be O(1) on every backend that ships with this
package (Hive uses Box.containsKey; SQL backends use an
indexed SELECT 1 ... LIMIT 1).
Implementation
@override
Future<bool> exists(String key) async => _db.raw.select(
'SELECT 1 FROM notifications WHERE id = ? LIMIT 1;', [key]).isNotEmpty;