feat(admin-web): surface generation error message and speed up active polling

Hoists hasActiveGenerationStatus into the shared display helper so all
views agree on what counts as in-flight generation, shows the backend
generation_error_message inline on the imitation list and detail drawer
when status is failed, and drops the workspace/templates/imitation poll
interval from 10s to 3s (with a 5s fallback when streaming is enabled)
plus explicit query-cache invalidation so failed/completed states show
up promptly.

apps/admin-web/src/components/ArticleGenerateStatus.vue

@@ -2,16 +2,14 @@
 import { LoadingOutlined } from '@ant-design/icons-vue'
 import { computed } from 'vue'
 
-import { getGenerateStatusMeta } from '@/lib/display'
+import { getGenerateStatusMeta, hasActiveGenerationStatus } from '@/lib/display'
 
 const props = defineProps<{
   status?: string | null
 }>()
 
 const statusMeta = computed(() => getGenerateStatusMeta(props.status))
-const isLoading = computed(
-  () => props.status === 'queued' || props.status === 'generating' || props.status === 'running',
-)
+const isLoading = computed(() => hasActiveGenerationStatus(props.status))
 </script>
 
 <template>