feat: implement backend API clients for multi-marketplace integration and frontend service layer

backend/avito_api/services/messenger.py

@@ -0,0 +1,29 @@
+from typing import Dict, Any, Optional
+from ..client import AvitoClient
+
+class MessengerService:
+    """Service to handle Avito Messenger (Chats)."""
+    
+    def __init__(self, client: AvitoClient):
+        self.client = client
+
+    def get_chats(self, user_id: str, limit: int = 100, offset: int = 0) -> Dict[str, Any]:
+        """
+        Get list of active chats for a user.
+        Endpoint: /messenger/v2/accounts/{user_id}/chats
+        """
+        params = {"limit": limit, "offset": offset}
+        return self.client.get(f"/messenger/v2/accounts/{user_id}/chats", params=params)
+
+    def send_message(self, user_id: str, chat_id: str, text: str) -> Dict[str, Any]:
+        """
+        Send a text message to a specific chat.
+        Endpoint: /messenger/v1/accounts/{user_id}/chats/{chat_id}/messages
+        """
+        payload = {
+            "message": {
+                "text": text
+            },
+            "type": "text"
+        }
+        return self.client.post(f"/messenger/v1/accounts/{user_id}/chats/{chat_id}/messages", data=payload)