feat(v3): T19-T20 模型池设置 UI + 智能体页 + 指标分账卡

- 设置页「模型池」区块：三角色指派下拉、条目表格（档位徽标/单价/启用/测试/编辑/删除）、
  内联添加表单（模型列表探测 datalist，api_key 留空保留原值）
- 新增「🤖 智能体」页 /agent：任务输入 + 池模型选择、工具调用时间轴
  （round/tool_call/tool_result/usage/final）、左侧工作区文件浏览与预览
- 指标页「按模型分账」卡片（v2.by_model 表格）
- 修复：SSE final 后 EventSource 自动重连回放导致的事件重复渲染（终态即关闭+回放拦截）

gateway/static/assets/MetricsView-DGq419lf.css

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