fix(knowledge): list_collections 增加异常保护与元数据重载

1. 遍历集合时 try/except 捕获 ChromaDB 异常（NotFoundError/InternalError），
   跳过失效集合并自动清理元数据，防止 /collections 500
2. 每次调用从磁盘重载 kb_metadata.json，确保多 worker 进程间状态一致，
   修复 GUNICORN_WORKERS>1 时返回数量波动问题

knowledge/collection.py

@@ -348,6 +348,8 @@ class CollectionMixin:
             - department: 所属部门
             - description: 描述
         """
+        # 从磁盘重新加载元数据，确保多 worker 进程间状态一致
+        self._metadata = self._load_metadata()
         result = []
 
         # 扫描 base_path 下的所有子目录作为向量库
@@ -382,16 +384,31 @@ class CollectionMixin:
 
         self._save_metadata()
 
+        stale_collections = []
+
         for name, info in self._metadata.get("collections", {}).items():
-            collection = self.get_collection(name)
-            result.append(CollectionInfo(
-                name=name,
-                display_name=info.get("display_name", name),
-                document_count=collection.count() if collection else 0,
-                created_at=info.get("created_at", ""),
-                department=info.get("department", ""),
-                description=info.get("description", "")
-            ))
+            try:
+                collection = self.get_collection(name)
+                result.append(CollectionInfo(
+                    name=name,
+                    display_name=info.get("display_name", name),
+                    document_count=collection.count() if collection else 0,
+                    created_at=info.get("created_at", ""),
+                    department=info.get("department", ""),
+                    description=info.get("description", "")
+                ))
+            except Exception as e:
+                logger.warning(
+                    f"跳过异常向量库 '{name}': {e}，可能是 ChromaDB 数据目录已丢失"
+                )
+                stale_collections.append(name)
+
+        # 清理元数据中指向已失效集合的条目
+        if stale_collections:
+            for name in stale_collections:
+                self._metadata.get("collections", {}).pop(name, None)
+                logger.info(f"清理失效向量库元数据: {name}")
+            self._save_metadata()
 
         return result