fix: 文件下载乱码 — 签名 URL 增加 Content-Disposition: attachment

- generate_presigned_url 支持 download 参数，添加 response-content-disposition
- sign-url API 新增 download 查询参数
- 前端 getSignedUrl 支持 download 模式
- 下载时传 download=true，浏览器触发文件保存而非显示乱码

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/upload.py

@@ -135,6 +135,7 @@ class SignedUrlResponse(BaseModel):
 async def get_signed_url(
     url: str = Query(..., description="文件的原始 URL 或 file_key"),
     expire: int = Query(3600, ge=60, le=43200, description="有效期（秒），默认1小时，最长12小时"),
+    download: bool = Query(False, description="是否强制下载（添加 Content-Disposition: attachment）"),
     current_user: User = Depends(get_current_user),
 ):
     """
@@ -157,7 +158,7 @@ async def get_signed_url(
         )
 
     try:
-        signed_url = generate_presigned_url(file_key, expire_seconds=expire)
+        signed_url = generate_presigned_url(file_key, expire_seconds=expire, download=download)
     except ValueError as e:
         raise HTTPException(
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,