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

backend/ae_api/services/transactions.py

@@ -0,0 +1,19 @@
+from typing import Dict, Any, Optional
+from ..client import AEClient
+
+class TransactionsService:
+    """Service to handle AE Platform Transactions (Orders)."""
+    
+    def __init__(self, client: AEClient):
+        self.client = client
+
+    def get_list(self, user_id: Optional[str] = None, params: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
+        """
+        Get list of transactions (orders) for a user.
+        Endpoint: GET /users/{user_id}/transactions
+        """
+        uid = user_id or self.client.user_id
+        if not uid:
+            raise ValueError("user_id must be provided either in the method call or client initialization.")
+            
+        return self.client.get(f"/users/{uid}/transactions", params=params)