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/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)
+}