refactor(background): enhance BackgroundTask and VisualRecognitionTaskProcessor structure and logging

Web/src/pages/admin/system/ConfigTabs.tsx

@@ -289,13 +289,13 @@ const ConfigTabs: React.FC<ConfigTabsProps> = ({
           isMobile={isMobile}
         >
           <Form form={formsMap.AppSettings} layout="vertical" size={isMobile ? "middle" : "large"}>
-            {renderConfigFormItems(formsMap.AppSettings, "AppSettings", ['ServerUrl'])}
+            {renderConfigFormItems(formsMap.AppSettings, "AppSettings", ['ServerUrl', 'MaxConcurrentTasks'])}
             <Divider style={{ margin: '12px 0 20px' }} />
             <Form.Item style={{ marginBottom: 0, textAlign: 'center' }}>
               <Button
                 type="primary"
                 icon={<SaveOutlined />}
-                onClick={() => onSaveAllForGroup(formsMap.AppSettings, "AppSettings", ['ServerUrl'])}
+                onClick={() => onSaveAllForGroup(formsMap.AppSettings, "AppSettings", ['ServerUrl', 'MaxConcurrentTasks'])}
                 style={{ width: isMobile ? '100%' : '240px' }}
               >
                 保存所有应用设置

Web/src/pages/admin/system/Index.tsx

@@ -43,7 +43,8 @@ const allDescriptions: Record<string, Record<string, string>> = {
     LinuxDoCallbackUrl: 'LinuxDo OAuth 认证回调地址'
   },
   AppSettings: {
-    ServerUrl: '服务器URL'
+    ServerUrl: '服务器URL',
+    MaxConcurrentTasks: '后台任务最大并发处理数量 (例如: 图像分析、标签生成等)'
   },
   Storage: {
     DefaultStorage: '已登录用户上传文件时的默认存储位置',