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>

backend/app/api/upload.py

@@ -21,11 +21,11 @@ class UploadPolicyRequest(BaseModel):
 
 
 class UploadPolicyResponse(BaseModel):
-    """COS 直传凭证响应"""
-    q_sign_algorithm: str
-    q_ak: str
-    q_key_time: str
-    q_signature: str
+    """TOS 直传凭证响应"""
+    x_tos_algorithm: str
+    x_tos_credential: str
+    x_tos_date: str
+    x_tos_signature: str
     policy: str
     host: str
     dir: str
@@ -56,9 +56,9 @@ async def get_upload_policy(
     current_user: User = Depends(get_current_user),
 ):
     """
-    获取 COS 直传凭证
+    获取 TOS 直传凭证
 
-    前端使用此凭证直接上传文件到腾讯云 COS，无需经过后端。
+    前端使用此凭证直接上传文件到火山引擎 TOS，无需经过后端。
 
     文件类型说明：
     - script: 脚本文档 (docx, pdf, xlsx, txt, pptx)
@@ -92,10 +92,10 @@ async def get_upload_policy(
         )
 
     return UploadPolicyResponse(
-        q_sign_algorithm=policy["q_sign_algorithm"],
-        q_ak=policy["q_ak"],
-        q_key_time=policy["q_key_time"],
-        q_signature=policy["q_signature"],
+        x_tos_algorithm=policy["x_tos_algorithm"],
+        x_tos_credential=policy["x_tos_credential"],
+        x_tos_date=policy["x_tos_date"],
+        x_tos_signature=policy["x_tos_signature"],
         policy=policy["policy"],
         host=policy["host"],
         dir=policy["dir"],