fix: honor account model mapping before group fallback

backend/internal/service/openai_model_mapping.go

@@ -0,0 +1,19 @@
+package service
+
+// resolveOpenAIForwardModel determines the upstream model for OpenAI-compatible
+// forwarding. Group-level default mapping only applies when the account itself
+// did not match any explicit model_mapping rule.
+func resolveOpenAIForwardModel(account *Account, requestedModel, defaultMappedModel string) string {
+	if account == nil {
+		if defaultMappedModel != "" {
+			return defaultMappedModel
+		}
+		return requestedModel
+	}
+
+	mappedModel, matched := account.ResolveMappedModel(requestedModel)
+	if !matched && defaultMappedModel != "" {
+		return defaultMappedModel
+	}
+	return mappedModel
+}