feat(generate): 生成管线改为异步，前端接入轮询进度

后端:
- POST /api/v1/generate 改为异步模式，立即返回 taskId
- 管线在后台 goroutine 执行，状态通过 GET /tasks/:taskId 轮询
- 任务状态流转: pending → running → saving → completed/failed

前端:
- generation store: submit 后每 2s 轮询 getTask，完成时自动 getAssets
- GeneratePage: 实时显示轮询状态文本 + 进度条
- ResultPage: 挂载时轮询任务，未完成显示骨架屏+进度，完成自动加载素材
- Types: Task.status 增加 submitted / saving 状态

frontend/src/pages/GeneratePage.tsx

@@ -21,6 +21,7 @@ export default function GeneratePage() {
     status,
     progress,
     taskId,
+    statusText,
     submit,
     reset: resetGeneration,
   } = useGenerationStore()
@@ -91,7 +92,7 @@ export default function GeneratePage() {
 
           {status === 'running' && (
             <p style={{ textAlign: 'center', color: 'var(--text-secondary)' }}>
-              管线执行中，请稍候...
+              {statusText || '管线执行中，请稍候...'}
             </p>
           )}
 

frontend/src/pages/ResultPage.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useState } from 'react'
+import { useEffect, useState, useRef } from 'react'
 import { Link, useParams } from 'react-router-dom'
 import { getTask, getAssets } from '../api/generate'
 import type { Asset, Task } from '../api/types'
@@ -10,19 +10,47 @@ export default function ResultPage() {
   const [task, setTask] = useState<Task | null>(null)
   const [assets, setAssets] = useState<Asset[]>([])
   const [loading, setLoading] = useState(true)
+  const [polling, setPolling] = useState(false)
+  const pollRef = useRef<ReturnType<typeof setInterval>>()
 
   useEffect(() => {
     if (!taskId) return
     setLoading(true)
-    Promise.all([getTask(taskId), getAssets(taskId)])
-      .then(([t, a]) => {
+
+    const fetchTask = async () => {
+      try {
+        const t = await getTask(taskId)
         setTask(t)
-        setAssets(a)
-      })
-      .finally(() => setLoading(false))
+
+        if (t.status === 'completed') {
+          if (pollRef.current) clearInterval(pollRef.current)
+          setPolling(false)
+          const a = await getAssets(taskId)
+          setAssets(a)
+          setLoading(false)
+        } else if (t.status === 'failed') {
+          if (pollRef.current) clearInterval(pollRef.current)
+          setPolling(false)
+          setLoading(false)
+        } else if (!pollRef.current) {
+          // 开始轮询
+          setPolling(true)
+          pollRef.current = setInterval(fetchTask, 2000)
+        }
+      } catch {
+        // 出错也停止加载态
+        setLoading(false)
+      }
+    }
+
+    fetchTask()
+
+    return () => {
+      if (pollRef.current) clearInterval(pollRef.current)
+    }
   }, [taskId])
 
-  if (loading) {
+  if (loading || polling) {
     return (
       <div className="container page-enter" style={{ paddingTop: 40 }}>
         <div className="card" style={{ marginBottom: 24 }}>
@@ -30,6 +58,13 @@ export default function ResultPage() {
           <div style={{ marginTop: 16 }}>
             <Skeleton variant="text" lines={4} />
           </div>
+          {task && (
+            <p style={{ textAlign: 'center', color: 'var(--text-secondary)', marginTop: 16 }}>
+              {task.status === 'pending' && '任务排队中...'}
+              {task.status === 'running' && `生成中... ${task.progress ?? 0}%`}
+              {task.status === 'submitted' && '已提交，等待处理...'}
+            </p>
+          )}
         </div>
         <div className="card">
           <Skeleton variant="card" />
@@ -53,27 +88,35 @@ export default function ResultPage() {
     <div className="container page-enter" style={{ paddingTop: 24, paddingBottom: 40 }}>
       <h1 style={{ fontSize: 24, marginBottom: 32 }}>生成结果</h1>
 
-      {/* 任务信息 */}
       <section className="card" style={{ marginBottom: 24 }}>
-        <h2 style={{ fontSize: 16, marginBottom: 16 }}>任务信息</h2>
-        <div
-          style={{
-            display: 'grid',
-            gridTemplateColumns: '120px 1fr',
-            gap: '8px 16px',
-            fontSize: 13,
-          }}
-        >
+        <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', marginBottom: 16 }}>
+          <h2 style={{ fontSize: 16, marginBottom: 0 }}>任务信息</h2>
+          <div style={{ display: 'flex', gap: 12 }}>
+            {assets.length > 0 && (
+              <button
+                className="btn-primary"
+                onClick={() => downloadAssets(assets)}
+                style={{ padding: '8px 20px', fontSize: 13 }}
+              >
+                下载全部素材
+              </button>
+            )}
+            <Link
+              to={`/projects/${projectId}/generate`}
+              className="btn-secondary"
+              style={{ padding: '8px 20px', fontSize: 13, borderRadius: 'var(--radius)', display: 'inline-block' }}
+            >
+              继续生成
+            </Link>
+          </div>
+        </div>
+        <div style={{ display: 'grid', gridTemplateColumns: '120px 1fr', gap: '8px 16px', fontSize: 13 }}>
           <span style={{ color: 'var(--text-secondary)' }}>提示词</span>
           <span>{task.prompt}</span>
           <span style={{ color: 'var(--text-secondary)' }}>素材类型</span>
           <span>{task.assetType}</span>
           <span style={{ color: 'var(--text-secondary)' }}>状态</span>
-          <span
-            style={{
-              color: task.status === 'completed' ? 'var(--success)' : 'var(--error)',
-            }}
-          >
+          <span style={{ color: task.status === 'completed' ? 'var(--success)' : 'var(--error)' }}>
             {task.status === 'completed' ? '已完成' : '失败'}
           </span>
           <span style={{ color: 'var(--text-secondary)' }}>创建时间</span>
@@ -93,29 +136,29 @@ export default function ResultPage() {
         </div>
       </section>
 
-      {/* 素材预览 */}
       <section className="card" style={{ marginBottom: 24 }}>
-        <div
-          style={{
-            display: 'flex',
-            justifyContent: 'space-between',
-            alignItems: 'center',
-            marginBottom: 16,
-          }}
-        >
-          <h2 style={{ fontSize: 16 }}>素材预览</h2>
-          {assets.length > 0 && (
-            <button
-              className="btn-primary"
-              onClick={() => alert('下载功能即将上线')}
-              style={{ padding: '8px 20px', fontSize: 13 }}
-            >
-              下载素材
-            </button>
-          )}
-        </div>
+        <h2 style={{ fontSize: 16, marginBottom: 16 }}>素材预览</h2>
         <AssetPreview assets={assets} />
       </section>
     </div>
   )
 }
+
+async function downloadAssets(assets: Asset[]) {
+  for (const a of assets) {
+    try {
+      const res = await fetch(a.url)
+      const blob = await res.blob()
+      const blobUrl = URL.createObjectURL(blob)
+      const link = document.createElement('a')
+      link.href = blobUrl
+      link.download = `${a.id}.${a.format}`
+      document.body.appendChild(link)
+      link.click()
+      document.body.removeChild(link)
+      URL.revokeObjectURL(blobUrl)
+    } catch {
+      window.open(a.url, '_blank')
+    }
+  }
+}