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/admin_dashboard.html

@@ -33,6 +33,7 @@
     <button class="admin-action-btn" onclick="adminAction('crawl')">🔄 抓取今天</button>
     <button class="admin-action-btn" onclick="adminAction('summarize')">📝 批量总结</button>
     <button class="admin-action-btn" onclick="adminAction('cleanup')">🧹 清理临时文件</button>
+    <button class="admin-action-btn" onclick="refreshUpvotes()">👍 刷新投票</button>
   </div>
 
   <div class="admin-info-grid">
@@ -59,6 +60,10 @@
           <span class="info-value">{{ stats.next_run[:19] | replace('T', ' ') }}</span>
         </div>
         {% endif %}
+        <div class="info-row">
+          <span class="info-label">投票刷新</span>
+          <span class="info-value">每日自动刷新最近 {{ stats.upvote_refresh_days | default(7) }} 天</span>
+        </div>
         {% if stats.active_locks %}
         <div class="info-row">
           <span class="info-label">活跃任务</span>
@@ -181,5 +186,12 @@
       .then(data => { if (data) showToast(data.error ? "❌ " + data.error.substring(0,200) : "✅ 流水线已触发"); })
       .catch(err => showToast("❌ 请求失败"));
   }
+
+  function refreshUpvotes() {
+    fetch("/admin/refresh-upvotes", { method: "POST", headers: { "Content-Type": "application/json" } })
+      .then(r => { if (r.status===303||r.status===401) { window.location.href="/admin/login"; return; } return r.json(); })
+      .then(data => { if (data) showToast(data.error ? "❌ " + data.error.substring(0,200) : `✅ 已刷新 ${data.updated || 0} 篇论文投票`); })
+      .catch(err => showToast("❌ 请求失败"));
+  }
 </script>
 {% endblock %}