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

backend/mm_api/services/shipments.py

@@ -0,0 +1,30 @@
+from typing import Dict, Any
+from ..client import MMClient
+
+class ShipmentsService:
+    """Service to handle Magnit Market Shipments."""
+    
+    def __init__(self, client: MMClient):
+        self.client = client
+
+    def get_list(self, dir: str = "ASC", page_size: int = 100, page_token: str = None, **filters) -> Dict[str, Any]:
+        """
+        Get list of shipments.
+        Endpoint: /api/seller/v1/shipments/list
+        """
+        payload = {
+            "dir": dir,
+            "page_size": page_size
+        }
+        if page_token:
+            payload["page_token"] = page_token
+            
+        payload.update(filters)
+        return self.client.post("/api/seller/v1/shipments/list", data=payload)
+
+    def confirm(self, shipment_id: str) -> Dict[str, Any]:
+        """
+        Confirm a shipment.
+        Endpoint: /api/seller/v1/shipments/confirm
+        """
+        return self.client.post("/api/seller/v1/shipments/confirm", data={"shipment_id": shipment_id})