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/index.html

@@ -23,16 +23,6 @@ endblock %} {% block content %}
 </div>
 {% endif %}
 
-<div class="date-quick-nav">
-  <span>有数据的日期：</span>
-  {% for d in available_dates[:10] %}
-  <a
-    href="/day/{{ d }}"
-    class="date-chip {% if d == current_date %}active{% endif %}"
-    >{{ d }}</a
-  >
-  {% endfor %}
-</div>
 {% endblock %}
 
 {% block scripts %}