feat: 腾讯云 COS 迁移至火山引擎 TOS 对象存储

签名算法从 COS HMAC-SHA1 改为 TOS V4 HMAC-SHA256，
更新前后端上传凭证字段、配置项、备份脚本和文档。

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

frontend/lib/api.ts

@@ -136,10 +136,10 @@ export interface RefreshTokenResponse {
 }
 
 export interface UploadPolicyResponse {
-  q_sign_algorithm: string
-  q_ak: string
-  q_key_time: string
-  q_signature: string
+  x_tos_algorithm: string
+  x_tos_credential: string
+  x_tos_date: string
+  x_tos_signature: string
   policy: string
   host: string
   dir: string
@@ -426,7 +426,7 @@ class ApiClient {
   // ==================== 文件上传 ====================
 
   /**
-   * 获取 COS 上传凭证
+   * 获取 TOS 上传凭证
    */
   async getUploadPolicy(fileType: string = 'general'): Promise<UploadPolicyResponse> {
     const response = await this.client.post<UploadPolicyResponse>('/upload/policy', {