Expose backend identity and capacity errors

internal/adapter/http/handler.go

@@ -125,6 +125,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			PromptHash:         res.PromptHash,
 			RenderedPromptHash: res.RenderedPromptHash,
 			SelectedProfileID:  res.SelectedProfileID,
+			SelectedBackendID:  res.SelectedBackendID,
 			SessionID:          res.SessionID,
 			ModelName:          res.ModelName,
 			Endpoint:           res.Endpoint,
@@ -173,6 +174,7 @@ func executionTargetOverrideFromModelOverrideDTO(dto *modelOverrideRequestDTO) *
 func modelParamsDTOFromExecutionTarget(target promptkit.ExecutionTarget) modelParamsDTO {
 	return modelParamsDTO{
 		Endpoint:        target.Endpoint,
+		BackendID:       target.BackendID,
 		Model:           target.Model,
 		Temperature:     target.Temperature,
 		MaxTokens:       target.MaxTokens,
@@ -220,6 +222,8 @@ func mapRunError(err error) (int, string, string) {
 		return http.StatusBadRequest, "artifact_read_failed", "failed to read input artifact"
 	case errors.Is(err, promptkit.ErrPromptRender):
 		return http.StatusBadRequest, "prompt_render_failed", "failed to render prompt"
+	case errors.Is(err, promptkit.ErrCapacityExceeded):
+		return http.StatusServiceUnavailable, "capacity_exceeded", "model backend capacity is exhausted"
 	case errors.Is(err, promptkit.ErrLLMGenerate):
 		return http.StatusBadGateway, "llm_failed", "model generation request failed"
 	case errors.Is(err, promptkit.ErrValidation):