feat: 接入七牛云存储，替换本地文件存储

- generate handler 中的本地文件写入替换为 storageSvc.Upload() 调用
- 移除 main.go 中的本地 generation 目录创建和静态文件服务
- 更新 api.md 接口状态标记

backend/internal/handler/generate.go

@@ -5,8 +5,6 @@ import (
 	"fmt"
 	"log"
 	"net/http"
-	"os"
-	"path/filepath"
 	"sync"
 	"time"
 
@@ -141,23 +139,16 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 
 	updateStatus(taskID, "saving", 80)
 
-	// 保存图片到 ../generation/{projectId}/{taskId}/
-	genDir := filepath.Join("..", "generation", projectID, taskID)
-	if err := os.MkdirAll(genDir, 0755); err != nil {
-		updateFailed(taskID, "创建输出目录失败: "+err.Error())
-		return
-	}
-
 	assets := make([]AssetResponse, len(output.Assets))
 	for i, a := range output.Assets {
-		filename := fmt.Sprintf("%d.%s", i, a.Format)
-		filePath := filepath.Join(genDir, filename)
-		if err := os.WriteFile(filePath, a.Data, 0644); err != nil {
-			updateFailed(taskID, "保存图片失败: "+err.Error())
+		key := fmt.Sprintf("generation/%s/%s/%d.%s", projectID, taskID, i, a.Format)
+		cdnURL, err := storageSvc.Upload(ctx, key, a.Data)
+		if err != nil {
+			updateFailed(taskID, "上传素材失败: "+err.Error())
 			return
 		}
 		assets[i] = AssetResponse{
-			URL:    fmt.Sprintf("/generation/%s/%s/%s", projectID, taskID, filename),
+			URL:    cdnURL,
 			Format: a.Format,
 		}
 	}