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/services/admin.py

@@ -106,4 +106,5 @@ def get_admin_stats(db: Session) -> dict:
         "next_run": next_run.isoformat() if next_run else None,
         "recent_logs": recent_logs,
         "active_locks": active_locks,
+        "upvote_refresh_days": settings.UPVOTE_REFRESH_DAYS,
     }