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

backend/avito_api/__init__.py

@@ -0,0 +1,15 @@
+from .client import AvitoClient
+from .services.items import ItemsService
+from .services.messenger import MessengerService
+
+class AvitoAPI:
+    """
+    Main entry point for Avito SDK.
+    Instantiates all services with a shared client.
+    """
+    def __init__(self, client_id: str = None, client_secret: str = None):
+        self.client = AvitoClient(client_id, client_secret)
+        
+        # Initialize services
+        self.items = ItemsService(self.client)
+        self.messenger = MessengerService(self.client)