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.wxml

@@ -75,7 +75,7 @@
     <view class="field" wx:if="{{result.detect.reason_tokens && result.detect.reason_tokens.length}}">
       <text class="field-label">风险关键词</text>
       <view class="chip-group">
-        <text class="tag" wx:for="{{result.detect.reason_tokens}}" wx:key="*this">{{item}}</text>
+        <text class="tag tag-danger" wx:for="{{result.detect.reason_tokens}}" wx:key="token" data-token="{{item.token}}" data-weight="{{item.weight}}" bindtap="showTokenWeight">{{item.token}}</text>
       </view>
     </view>