fix: 修复 AI Code Review v5（轮询请求重叠+重播黑屏）

- useStep6Cover: setInterval 改为递归 setTimeout，确保上一次请求完成后再发起下一次，避免请求风暴
- useCanvasPlayer play(): 检测重播场景（currentTime 接近0但 decodedSegmentsRef 非空）时清空标记并递增 generation，让 decodeAroundPosition 重新解码

apps/web/src/pages/generate/hooks/useCanvasPlayer.ts

@@ -749,6 +749,13 @@ export function useCanvasPlayer(
   const play = useCallback(async () => {
     if (!state.hasSupport || isDestroyedRef.current) return
 
+    // 重播场景：currentTime 已回到起点但 decodedSegmentsRef 仍有旧标记
+    // 此时 FrameQueue 中旧帧已被淘汰，需清空标记让 decodeAroundPosition 重新解码
+    if (state.currentTime <= 0.1 && decodedSegmentsRef.current.size > 0) {
+      decodeGenerationRef.current++
+      decodedSegmentsRef.current.clear()
+    }
+
     setState((s) => ({ ...s, isPlaying: true }))
     playStartRef.current = performance.now()
     playStartOffsetRef.current = state.currentTime

apps/web/src/pages/generate/hooks/useStep6Cover.ts

@@ -137,38 +137,34 @@ export function useStep6Cover({
             asset_ids: assetIds,
             duration: duration || 30,
           })
-          // 轮询等待预览渲染完成，双重超时保护
-          const maxPolls = 60
-          let pollCount = 0
+          // 轮询等待预览渲染完成：递归 setTimeout 避免请求重叠 + 120s 超时兜底
           await new Promise<void>((resolve, reject) => {
+            let finished = false
+            const done = (fn: () => void) => {
+              if (finished) return
+              finished = true
+              clearTimeout(timeoutId)
+              fn()
+            }
             const timeoutId = setTimeout(() => {
-              clearInterval(poll)
-              reject(new Error("预览生成超时，请稍后重试"))
+              done(() => reject(new Error("预览生成超时，请稍后重试")))
             }, 120_000)
-            const poll = setInterval(async () => {
-              pollCount++
+            const poll = async () => {
+              if (finished) return
               try {
                 const status = await getPreviewStatus(previewResp.task_id)
                 if (status.status === "completed") {
-                  clearTimeout(timeoutId)
-                  clearInterval(poll)
-                  resolve()
+                  done(() => resolve())
                 } else if (status.status === "failed") {
-                  clearTimeout(timeoutId)
-                  clearInterval(poll)
-                  reject(new Error(status.error_message || "预览渲染失败"))
-                }
-                if (pollCount >= maxPolls) {
-                  clearTimeout(timeoutId)
-                  clearInterval(poll)
-                  reject(new Error("预览生成超时，请稍后重试"))
+                  done(() => reject(new Error(status.error_message || "预览渲染失败")))
+                } else {
+                  setTimeout(poll, 2000)
                 }
               } catch (e) {
-                clearTimeout(timeoutId)
-                clearInterval(poll)
-                reject(e)
+                done(() => reject(e))
               }
-            }, 2000)
+            }
+            poll()
           })
           message.success("预览视频就绪，重新生成封面...")
           // 重试封面生成