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

backend/flowwow_api/exceptions.py

@@ -0,0 +1,19 @@
+class FlowwowAPIError(Exception):
+    """Base exception for all Flowwow API errors."""
+    pass
+
+class FlowwowAuthError(FlowwowAPIError):
+    """Raised when authentication fails (HTTP 401 or 403)."""
+    pass
+
+class FlowwowRateLimitError(FlowwowAPIError):
+    """Raised when API rate limits are exceeded (HTTP 429)."""
+    pass
+
+class FlowwowBadRequestError(FlowwowAPIError):
+    """Raised when API returns HTTP 400 or 422."""
+    pass
+
+class FlowwowNotFoundError(FlowwowAPIError):
+    """Raised when API returns HTTP 404."""
+    pass