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/config.py

@@ -41,6 +41,7 @@ class Settings(BaseSettings):
     SCHEDULE_HOUR: int = 4
     SCHEDULE_MINUTE: int = 0
     APP_WORKERS: int = 1
+    UPVOTE_REFRESH_DAYS: int = 7  # 刷新最近 N 天论文的 upvotes
 
     # 数据库
     DATABASE_URL: str = "sqlite:///data/db/papers.db"