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/utils.py

@@ -57,6 +57,13 @@ def yesterday_str() -> str:
     return yesterday.isoformat()
 
 
+def recent_date_strs(n: int) -> list[str]:
+    """最近 N 天的日期字符串列表（含今天，按 APP_TIMEZONE）。"""
+    tz = ZoneInfo(settings.APP_TIMEZONE)
+    today = datetime.now(tz).date()
+    return [(today - timedelta(days=i)).isoformat() for i in range(n)]
+
+
 def latest_paper_date(db) -> str:
     """查询数据库中最新的 paper_date，无数据时回退到 today_str()。"""
     from sqlalchemy import func, select