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

backend/mega_api/__init__.py

@@ -0,0 +1,15 @@
+from .client import MegaClient
+from .services.orders import OrdersService
+from .services.assortment import AssortmentService
+
+class MegaMarketAPI:
+    """
+    Main entry point for MegaMarket SDK.
+    Instantiates all services with a shared client.
+    """
+    def __init__(self, api_token: str = None):
+        self.client = MegaClient(api_token)
+        
+        # Initialize services
+        self.orders = OrdersService(self.client)
+        self.assortment = AssortmentService(self.client)