feat: refactor PDF extraction to caption-based screenshots, add upvote refresh, clean up UI

- PDF extractor: rewrite from embedded bitmap extraction to caption-based
  page region screenshots. Finds Figure/Table captions via regex,截取上方/下方
  page region, handles compound figures and vector graphics.
- Upvote refresh: new crawler.refresh_upvotes() re-fetches upvotes for recent
  N days without inserting new papers. Scheduler runs daily 30min after pipeline.
- Admin: add /admin/refresh-upvotes endpoint and dashboard button.
- UI: remove date quick nav, show upvote update time on detail/card pages,
  clean up CSS date-chip styles.
- Utils: add recent_date_strs() helper.

app/templates/detail.html

@@ -22,6 +22,9 @@ endblock %} {% block content %}
       >📅 {{ paper.published_at or paper.paper_date }}</span
     >
     <span class="detail-upvotes">👍 {{ paper.upvotes }}</span>
+    {% if paper.crawled_at %}
+    <span class="detail-upvote-time" title="投票数据更新时间">{{ paper.crawled_at.strftime('%m-%d %H:%M') }}</span>
+    {% endif %}
   </div>
 
   {# 标签 #} {% if paper.tags %}