feat: 完善知识库数据模型和服务

- 添加知识库更多字段配置
- 优化知识库服务逻辑
- 添加文档解析相关接口

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

server/internal/handler/knowledge_handler.go

@@ -147,6 +147,10 @@ func (h *KnowledgeHandler) UploadDocument(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"success": false, "message": err.Error()})
 		return
 	}
+	if doc == nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"success": false, "message": "Upload failed"})
+		return
+	}
 
 	c.JSON(http.StatusOK, gin.H{
 		"success":  true,