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

backend/avito_api/services/items.py

@@ -0,0 +1,15 @@
+from typing import Dict, Any, Optional
+from ..client import AvitoClient
+
+class ItemsService:
+    """Service to handle Avito Items (Listings)."""
+    
+    def __init__(self, client: AvitoClient):
+        self.client = client
+
+    def get_info(self, item_id: str) -> Dict[str, Any]:
+        """
+        Get info about a specific item.
+        Endpoint: /core/v1/items/{item_id}
+        """
+        return self.client.get(f"/core/v1/items/{item_id}")