feat: 风险关键词红色标记 + 点击显示权重贡献

- 后端: _extract_reason_tokens 返回 [{token, weight}] 格式
- 前端: detect/batch 页面风险关键词使用红色标签样式
- 点击关键词弹窗显示权重值及判定倾向

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

miniprogram/pages/detect/index.js

@@ -98,5 +98,18 @@ Page({
 
   gotoHistory() {
     wx.navigateTo({ url: '/pages/history/index' })
+  },
+
+  showTokenWeight(e) {
+    const token = e.currentTarget.dataset.token
+    const weight = e.currentTarget.dataset.weight
+    const weightNum = Number(weight || 0)
+    const direction = weightNum >= 0 ? '倾向垃圾判定' : '倾向正常判定'
+    wx.showModal({
+      title: '关键词权重',
+      content: `关键词"${token}"\n权重贡献：${weightNum >= 0 ? '+' : ''}${weightNum.toFixed(4)}\n(${direction})`,
+      showCancel: false,
+      confirmText: '关闭'
+    })
   }
 })