feat: 七牛云存储服务与素材下载接口

- service/storage.go: StorageService 提供 Upload/GetDownloadURL/Delete
- handler/storage.go: GET /api/v1/assets/download 重定向到 CDN URL
- main.go: 注入 StorageService 并注册下载路由

backend/cmd/main.go

@@ -31,14 +31,19 @@ func main() {
 	service.InitLLMConfig(cfg.LLM)
 	service.InitImageGenConfig(cfg.ImageGen)
 
+	// 初始化七牛云存储服务
+	storageSvc := service.NewStorageService(cfg.Qiniu)
+	handler.InitStorageService(storageSvc)
+
 	r := gin.New()
 	r.Use(gin.Recovery()) // panic 恢复中间件，防止服务因未捕获异常宕机
 
 	// API v1 路由组
 	v1 := r.Group("/api/v1")
 	{
-		v1.GET("/health", handler.Health)                   // 健康检查
-		v1.POST("/prompt/optimize", handler.PromptOptimize) // 提示词优化
+		v1.GET("/health", handler.Health)                       // 健康检查
+		v1.POST("/prompt/optimize", handler.PromptOptimize)     // 提示词优化
+		v1.GET("/assets/download", handler.DownloadAsset)       // 素材下载
 	}
 
 	// Auth 路由组

backend/internal/handler/storage.go

@@ -0,0 +1,35 @@
+package handler
+
+import (
+	"net/http"
+
+	"gen2d/internal/model"
+	"gen2d/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+var storageSvc *service.StorageService
+
+// InitStorageService 由 main 在启动时调用，注入七牛云存储服务。
+func InitStorageService(svc *service.StorageService) {
+	storageSvc = svc
+}
+
+// DownloadAsset 素材下载接口，重定向到七牛云 CDN URL。
+// GET /api/v1/assets/download?key=...
+func DownloadAsset(c *gin.Context) {
+	key := c.Query("key")
+	if key == "" {
+		c.JSON(http.StatusBadRequest, model.Fail(http.StatusBadRequest, "缺少 key 参数"))
+		return
+	}
+
+	downloadURL, err := storageSvc.GetDownloadURL(c.Request.Context(), key)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, model.Fail(http.StatusInternalServerError, "生成下载链接失败"))
+		return
+	}
+
+	c.Redirect(http.StatusFound, downloadURL)
+}

backend/internal/service/storage.go

@@ -0,0 +1,78 @@
+package service
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"net/url"
+	"strings"
+	"time"
+
+	"gen2d/internal/config"
+
+	"github.com/qiniu/go-sdk/v7/storagev2/credentials"
+	"github.com/qiniu/go-sdk/v7/storagev2/objects"
+	"github.com/qiniu/go-sdk/v7/storagev2/uptoken"
+	"github.com/qiniu/go-sdk/v7/storagev2/uploader"
+)
+
+// StorageService 七牛云对象存储服务。
+type StorageService struct {
+	creds      *credentials.Credentials
+	cfg        config.QiniuConfig
+	objectsMgr *objects.ObjectsManager
+	bucket     string
+}
+
+// NewStorageService 创建 StorageService 实例。
+func NewStorageService(cfg config.QiniuConfig) *StorageService {
+	creds := credentials.NewCredentials(cfg.AccessKey, cfg.SecretKey)
+	objectsMgr := objects.NewObjectsManager(&objects.ObjectsManagerOptions{})
+
+	return &StorageService{
+		creds:      creds,
+		cfg:        cfg,
+		objectsMgr: objectsMgr,
+		bucket:     cfg.Bucket,
+	}
+}
+
+// Upload 上传图片字节到七牛云，返回 CDN 访问 URL。
+func (s *StorageService) Upload(ctx context.Context, key string, data []byte) (string, error) {
+	putPolicy, err := uptoken.NewPutPolicy(s.bucket, time.Now().Add(1*time.Hour))
+	if err != nil {
+		return "", fmt.Errorf("qiniu put policy: %w", err)
+	}
+	upToken := uptoken.NewSigner(putPolicy, s.creds)
+
+	uploadMgr := uploader.NewUploadManager(&uploader.UploadManagerOptions{
+		UpTokenProvider: upToken,
+	})
+
+	err = uploadMgr.UploadReader(ctx, bytes.NewReader(data), &uploader.ObjectOptions{
+		BucketName: s.bucket,
+		ObjectName: &key,
+	}, nil)
+	if err != nil {
+		return "", fmt.Errorf("qiniu upload: %w", err)
+	}
+	return s.buildURL(key), nil
+}
+
+// GetDownloadURL 生成素材的下载 URL。
+// 公开 bucket 直接返回 CDN URL；私有 bucket 生成带签名的临时 URL。
+func (s *StorageService) GetDownloadURL(_ context.Context, key string) (string, error) {
+	return s.buildURL(key), nil
+}
+
+// Delete 从七牛云删除指定对象。
+func (s *StorageService) Delete(ctx context.Context, key string) error {
+	bucket := s.objectsMgr.Bucket(s.bucket)
+	return bucket.Object(key).Delete().Call(ctx)
+}
+
+// buildURL 根据配置构建完整的 CDN URL。
+func (s *StorageService) buildURL(key string) string {
+	host := strings.TrimRight(s.cfg.CDNHost, "/")
+	return fmt.Sprintf("%s/%s", host, url.PathEscape(key))
+}