refactor: Projektstruktur in app/ und backend/ aufgeteilt

app/lib/entities/queued_message_entity.dart

@@ -0,0 +1,27 @@
+import 'package:objectbox/objectbox.dart';
+
+@Entity()
+class QueuedMessageEntity {
+  @Id()
+  int id = 0;
+
+  @Unique()
+  String messageId;
+
+  String topic;
+
+  String payload; // JSON-encoded payload
+
+  @Property(type: PropertyType.date)
+  DateTime createdAt;
+
+  int retryCount;
+
+  QueuedMessageEntity({
+    required this.messageId,
+    required this.topic,
+    required this.payload,
+    required this.createdAt,
+    this.retryCount = 0,
+  });
+}