Fix lint hint "Static method should be a constructor"

Author: dan-blank

lib/hive/service/database_service.dart

@@ -8,16 +8,13 @@ class DatabaseService {
   // Private constructor
   DatabaseService._();
 
+  static final DatabaseService _instance = DatabaseService._();
+
+  static DatabaseService get instance => _instance;
+
   static const String alarmBoxName = 'alarm_actions';
-  static DatabaseService? _instance;
   late Box<AlarmAction> _alarmBox;
 
-  // Singleton instance getter
-  static DatabaseService get instance {
-    _instance ??= DatabaseService._();
-    return _instance!;
-  }
-
   ValueListenable<Box<AlarmAction>> get alarmBoxListenable =>
       _alarmBox.listenable();