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

backend/ozon_api/__init__.py

@@ -0,0 +1,15 @@
+from .client import OzonClient
+from .services.products import ProductsService
+from .services.fbs import FBSService
+
+class OzonAPI:
+    """
+    Main entry point for Ozon SDK.
+    Instantiates all services with a shared client.
+    """
+    def __init__(self, client_id: str = None, api_key: str = None):
+        self.client = OzonClient(client_id, api_key)
+        
+        # Initialize services
+        self.products = ProductsService(self.client)
+        self.fbs = FBSService(self.client)