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/stores/generation.ts

@@ -1,6 +1,6 @@
 import { create } from 'zustand'
-import type { Asset, GenerateRequest, GenerateResponse } from '../api/types'
-import { submitGenerate } from '../api/generate'
+import type { Asset, GenerateRequest } from '../api/types'
+import { submitGenerate, getTask, getAssets } from '../api/generate'
 
 type Status = 'idle' | 'submitting' | 'running' | 'completed' | 'failed'
 
@@ -9,66 +9,99 @@ interface GenerationState {
   projectId: string | null
   progress: number
   status: Status
+  statusText: string
   assets: Asset[]
   error: string | null
   submit: (req: GenerateRequest) => Promise<void>
   reset: () => void
 }
 
-export const useGenerationStore = create<GenerationState>((set) => ({
+let pollTimer: ReturnType<typeof setInterval> | null = null
+
+function stopPolling() {
+  if (pollTimer) {
+    clearInterval(pollTimer)
+    pollTimer = null
+  }
+}
+
+export const useGenerationStore = create<GenerationState>((set, get) => ({
   taskId: null,
   projectId: null,
   progress: 0,
   status: 'idle',
+  statusText: '',
   assets: [],
   error: null,
 
   submit: async (req) => {
-    set({ status: 'submitting', error: null })
+    stopPolling()
+    set({ status: 'submitting', error: null, statusText: '提交中...' })
     try {
-      set({ status: 'running', progress: 30 })
-      const result = await submitGenerate(req)
-
-      set({ progress: 80 })
-      const assets = mapAssets(result)
+      const { taskId } = await submitGenerate(req)
 
       set({
-        taskId: result.taskId,
+        taskId,
         projectId: req.projectId,
-        status: 'completed',
-        progress: 100,
-        assets,
+        status: 'running',
+        progress: 10,
+        statusText: '任务已提交，等待生成...',
       })
+
+      // 开始轮询进度
+      pollTimer = setInterval(async () => {
+        try {
+          const task = await getTask(taskId)
+
+          set({
+            progress: task.progress ?? get().progress,
+            statusText:
+              task.status === 'running'
+                ? '生成中...'
+                : task.status === 'pending'
+                  ? '排队中...'
+                  : task.status,
+          })
+
+          if (task.status === 'completed') {
+            stopPolling()
+            set({ progress: 90, statusText: '获取结果...' })
+
+            const assets = await getAssets(taskId)
+            set({
+              status: 'completed',
+              progress: 100,
+              statusText: '生成完成',
+              assets,
+            })
+          } else if (task.status === 'failed') {
+            stopPolling()
+            set({
+              status: 'failed',
+              error: task.error || '生成失败',
+              statusText: '生成失败',
+            })
+          }
+        } catch {
+          // 网络错误不中断轮询
+        }
+      }, 2000)
     } catch (err) {
-      const errMsg = (err as Error).message
-      set({ status: 'failed', error: errMsg })
+      stopPolling()
+      set({ status: 'failed', error: (err as Error).message, statusText: '提交失败' })
     }
   },
 
   reset: () => {
+    stopPolling()
     set({
       taskId: null,
       projectId: null,
       progress: 0,
       status: 'idle',
+      statusText: '',
       assets: [],
       error: null,
     })
   },
 }))
-
-function mapAssets(resp: GenerateResponse): Asset[] {
-  return resp.assets.map((a, i) => ({
-    id: `asset-${i}`,
-    url: a.url,
-    format: a.format,
-    width: resp.metadata.frameWidth,
-    height: resp.metadata.frameHeight,
-    metadata: {
-      frameWidth: resp.metadata.frameWidth,
-      frameHeight: resp.metadata.frameHeight,
-      frameCount: resp.metadata.frameCount,
-      directions: resp.metadata.directions,
-    },
-  }))
-}