pos changes

lib/data/local/app_database.dart

@@ -524,4 +524,14 @@ class MetaKeys {
   static const String printerName = 'printer_name';
   static const String autoPrint = 'auto_print';
   static const String openDrawer = 'open_cash_drawer';
+
+  /// PIN that authorises taking a rung item back off a bill.
+  ///
+  /// Stored hashed, like a staff PIN — the terminal only ever holds the hash
+  /// and its salt, so lifting the database file does not hand over the ability
+  /// to void. Deliberately separate from staff PINs: an admin sets it once and
+  /// gives it to whoever is on the counter, so a removal can be authorised
+  /// without an admin walking over to the till.
+  static const String voidPinHash = 'void_pin_hash';
+  static const String voidPinSalt = 'void_pin_salt';
 }