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>

frontend/app/agency/briefs/[id]/page.tsx

@@ -417,16 +417,8 @@ export default function BriefConfigPage() {
       return
     }
     try {
-      const signedUrl = await api.getSignedUrl(file.url)
-      // 使用 a 标签触发下载
-      const a = document.createElement('a')
-      a.href = signedUrl
-      a.target = '_blank'
-      a.rel = 'noopener noreferrer'
-      a.download = file.name
-      document.body.appendChild(a)
-      a.click()
-      document.body.removeChild(a)
+      const signedUrl = await api.getSignedUrl(file.url, 3600, true)
+      window.open(signedUrl, '_blank')
     } catch {
       toast.error('获取下载链接失败')
     }
@@ -668,15 +660,8 @@ export default function BriefConfigPage() {
       return
     }
     try {
-      const signedUrl = await api.getSignedUrl(file.url)
-      const a = document.createElement('a')
-      a.href = signedUrl
-      a.target = '_blank'
-      a.rel = 'noopener noreferrer'
-      a.download = file.name
-      document.body.appendChild(a)
-      a.click()
-      document.body.removeChild(a)
+      const signedUrl = await api.getSignedUrl(file.url, 3600, true)
+      window.open(signedUrl, '_blank')
     } catch {
       toast.error('获取下载链接失败')
     }

frontend/lib/api.ts

@@ -473,10 +473,10 @@ class ApiClient {
   /**
    * 获取私有桶文件的预签名访问 URL
    */
-  async getSignedUrl(url: string, expire: number = 3600): Promise<string> {
+  async getSignedUrl(url: string, expire: number = 3600, download: boolean = false): Promise<string> {
     const response = await this.client.get<{ signed_url: string; expire_seconds: number }>(
       '/upload/sign-url',
-      { params: { url, expire } }
+      { params: { url, expire, download } }
     )
     return response.data.signed_url
   }