chore: 元数据并发安全增强与重排序配置更新

- knowledge: 写操作方法增加元数据重载，manager 增加 fcntl 文件锁
- config/engine: 重排序默认后端切换为云端，新增云端配置项

knowledge/manager.py

@@ -29,6 +29,11 @@
 import os
 import json
 import threading
+try:
+    import fcntl
+    _HAS_FCNTL = True
+except ImportError:
+    _HAS_FCNTL = False  # Windows 环境无 fcntl
 from typing import List, Dict, Optional, Tuple
 from pathlib import Path
 import logging
@@ -134,22 +139,36 @@ class KnowledgeBaseManager(
         logger.info(f"知识库管理器初始化完成，路径: {self.base_path}，发现 {len(existing_kbs)} 个向量库: {existing_kbs}")
 
     def _load_metadata(self) -> dict:
-        """加载元数据"""
+        """加载元数据（带文件锁，确保多 worker 进程间一致）"""
         metadata_path = os.path.join(self.base_path, KB_METADATA_FILE)
         if os.path.exists(metadata_path):
             try:
                 with open(metadata_path, 'r', encoding='utf-8') as f:
-                    return json.load(f)
+                    if _HAS_FCNTL:
+                        fcntl.flock(f.fileno(), fcntl.LOCK_SH)
+                    try:
+                        return json.load(f)
+                    finally:
+                        if _HAS_FCNTL:
+                            fcntl.flock(f.fileno(), fcntl.LOCK_UN)
             except Exception as e:
                 logger.error(f"加载元数据失败: {e}")
         return {"collections": {}}
 
     def _save_metadata(self):
-        """保存元数据"""
+        """保存元数据（带文件锁，防止并发写入数据覆盖）"""
         metadata_path = os.path.join(self.base_path, KB_METADATA_FILE)
         try:
             with open(metadata_path, 'w', encoding='utf-8') as f:
-                json.dump(self._metadata, f, ensure_ascii=False, indent=2)
+                if _HAS_FCNTL:
+                    fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+                try:
+                    json.dump(self._metadata, f, ensure_ascii=False, indent=2)
+                    f.flush()
+                    os.fsync(f.fileno())
+                finally:
+                    if _HAS_FCNTL:
+                        fcntl.flock(f.fileno(), fcntl.LOCK_UN)
         except Exception as e:
             logger.error(f"保存元数据失败: {e}")