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

backend/wb_api/exceptions.py

@@ -0,0 +1,15 @@
+class WBAPIError(Exception):
+    """Base exception for all Wildberries API errors."""
+    pass
+
+class WBAuthError(WBAPIError):
+    """Raised when authentication fails (HTTP 401)."""
+    pass
+
+class WBRateLimitError(WBAPIError):
+    """Raised when API rate limits are exceeded (HTTP 429)."""
+    pass
+
+class WBBadRequestError(WBAPIError):
+    """Raised when API returns HTTP 400 (Bad Request)."""
+    pass