refactor: Projektstruktur in app/ und backend/ aufgeteilt

app/lib/models/message_envelope.dart

@@ -0,0 +1,90 @@
+/// Message Envelope for wrapping all WebSocket messages with metadata
+/// 
+/// This provides reliable message delivery with acknowledgment mechanism
+class MessageEnvelope {
+  /// Unique message identifier (UUID)
+  final String messageId;
+
+  /// Timestamp when the message was created
+  final DateTime timestamp;
+
+  /// Target topic
+  final String topic;
+
+  /// Original message payload (can be Map or List)
+  final dynamic payload;
+
+  /// Whether this message requires acknowledgment
+  final bool requiresAck;
+
+  /// Number of retry attempts (for tracking)
+  final int retryCount;
+
+  /// Optional expiration timestamp
+  final DateTime? expiresAt;
+
+  MessageEnvelope({
+    required this.messageId,
+    required this.timestamp,
+    required this.topic,
+    required this.payload,
+    this.requiresAck = true,
+    this.retryCount = 0,
+    this.expiresAt,
+  });
+
+  /// Create MessageEnvelope from JSON
+  factory MessageEnvelope.fromJson(Map<String, dynamic> json) {
+    return MessageEnvelope(
+      messageId: json['messageId'] as String,
+      timestamp: DateTime.parse(json['timestamp'] as String),
+      topic: json['topic'] as String,
+      payload: json['payload'],
+      requiresAck: json['requiresAck'] as bool? ?? true,
+      retryCount: json['retryCount'] as int? ?? 0,
+      expiresAt: json['expiresAt'] != null
+          ? DateTime.parse(json['expiresAt'] as String)
+          : null,
+    );
+  }
+
+  /// Convert MessageEnvelope to JSON
+  Map<String, dynamic> toJson() {
+    return {
+      'messageId': messageId,
+      'timestamp': timestamp.toIso8601String(),
+      'topic': topic,
+      'payload': payload,
+      'requiresAck': requiresAck,
+      'retryCount': retryCount,
+      if (expiresAt != null) 'expiresAt': expiresAt!.toIso8601String(),
+    };
+  }
+
+  /// Create a copy with updated fields
+  MessageEnvelope copyWith({
+    String? messageId,
+    DateTime? timestamp,
+    String? topic,
+    dynamic payload,
+    bool? requiresAck,
+    int? retryCount,
+    DateTime? expiresAt,
+  }) {
+    return MessageEnvelope(
+      messageId: messageId ?? this.messageId,
+      timestamp: timestamp ?? this.timestamp,
+      topic: topic ?? this.topic,
+      payload: payload ?? this.payload,
+      requiresAck: requiresAck ?? this.requiresAck,
+      retryCount: retryCount ?? this.retryCount,
+      expiresAt: expiresAt ?? this.expiresAt,
+    );
+  }
+
+  @override
+  String toString() {
+    return 'MessageEnvelope(messageId: $messageId, topic: $topic, requiresAck: $requiresAck, retryCount: $retryCount)';
+  }
+}
+