feat: 添加监控仪表盘

- 新增 Lit.js 组件化 UI (ui/ 目录)
  - tasks-chart: 带十字准星和拖拽选择的图表
  - queue-table: 队列列表，支持暂停/恢复
  - queue-modal: 队列详情弹窗，支持任务重试
  - time-range-picker: Prometheus 风格时间选择器
  - help-tooltip: 可复用的提示组件

- HTTPHandler 功能
  - SSE 实时推送 (stats + queues)
  - 队列暂停/恢复 API
  - 任务重试 API
  - 时间范围查询 API

- Inspector 改进
  - Prometheus 风格单表存储
  - 集成到 Start/Stop 生命周期
  - 新增 PauseQueue/UnpauseQueue/RunTask 方法

- 代码重构
  - Start 函数拆分为小函数
  - 优雅关闭流程优化

- 其他
  - 忽略 SQLite 数据库文件
  - example 添加延迟/定点任务示例

ui/components/help-tooltip.js

@@ -0,0 +1,80 @@
+import { LitElement, html, css } from 'lit';
+
+class HelpTooltip extends LitElement {
+  static properties = {
+    text: { type: String }
+  };
+
+  static styles = css`
+    :host {
+      display: inline-flex;
+      align-items: center;
+      justify-content: center;
+      position: relative;
+    }
+
+    .icon {
+      display: inline-flex;
+      align-items: center;
+      justify-content: center;
+      width: 14px;
+      height: 14px;
+      border-radius: 50%;
+      background: #616161;
+      color: #9e9e9e;
+      font-size: 10px;
+      cursor: help;
+    }
+
+    .icon:hover {
+      background: #757575;
+      color: #e0e0e0;
+    }
+
+    .icon:hover + .tooltip {
+      display: block;
+    }
+
+    .tooltip {
+      display: none;
+      position: absolute;
+      top: 100%;
+      left: 50%;
+      transform: translateX(-50%);
+      margin-top: 8px;
+      padding: 8px 12px;
+      background: #212121;
+      color: #e0e0e0;
+      font-size: 12px;
+      font-weight: normal;
+      border-radius: 4px;
+      white-space: nowrap;
+      z-index: 1000;
+      box-shadow: 0 2px 8px rgba(0,0,0,0.3);
+    }
+
+    .tooltip::before {
+      content: '';
+      position: absolute;
+      bottom: 100%;
+      left: 50%;
+      transform: translateX(-50%);
+      border: 6px solid transparent;
+      border-bottom-color: #212121;
+    }
+  `;
+
+  constructor() {
+    super();
+    this.text = '';
+  }
+
+  render() {
+    return html`
+      <span class="icon">?</span>
+      <span class="tooltip">${this.text}</span>
+    `;
+  }
+}
+
+customElements.define('help-tooltip', HelpTooltip);